Only

Only (pronounced ohn-lee)

Adverb

(1) Without others or anything further; alone; solely; exclusively.

(2) No more than; merely; just.

(3) As recently as.

(4) In the final outcome or decision.

Adjective

(5) Being the single one or the relatively few of the kind.

(6) Having no sibling or (less common) no sibling of the same sex (also a noun in this context).

(7) Mere (obsolete).

(8) Single in superiority or distinction; unique; the best.

Conjunction

(9) But (introducing a single restriction, restraining circumstance, or the like).

(10) Except (frowned upon by some).

Pre 900: From the Middle English oonly, onli, onlych, onelich & anely, from the Old English ānlich, ānlīc & ǣnlich (like; similar; equal; unique, solitary, literally "one-like”), from the Proto-Germanic ainalīkaz (one + -ly).  It was cognate with the Old Frisian einlik, the obsolete Dutch eenlijk, the German ähnlich (similar), the Old Norse álíkr, the Old High German einlih, the Danish einlig and the Swedish enlig (unified).  Synonyms include solitary & lone in one context and peerless & exclusive in the other.  Only is a noun, adjective, adverb & conjunction, onliness, onlyer & onlier are nouns and onliest & onlest are adjectives ; the noun plural is either onlys or onlies (both rarely used).

Only’s use as an adverb (alone, no other or others than; in but one manner; for but one purpose) and a conjunction (but, except) developed in Middle English.  In English, the familiar distinction of only and alone (now usually in reference to emotional states) is unusual; in many languages the same word serves for both although Modern German has the distinction in allein/einzig.  The mid fifteenth century phrase "only-begotten" is biblical, translating Latin unigenitus and Greek monogenes; the Old English word was ancenned. The term "only child" has been in use since at least the early eighteenth century.  The derived forms were once in more frequent use than now.  Someone who only adheres to the particular thing mentioned, excluding any alternatives. Onlyism (definitely non-standard) used to be quite a thing in Christianity in matters where there were different versions of documents and among Church of England congregations (often in the same parish) some were once adamant that only a certain edition of the Book of Common Prayer was acceptable and the others represented revisionism, heresy or, worse of all, smelled of popery.  Thus there were 1549-onlyiers, 1559-onlyiers, 1562-onlyiers etc.  The same factionalism of course continues to exist in many religions (and in secular movements and institutions too) but onlier has faded from use.  The adjectives onliest & onlest (a superlative form of only used almost exclusively in the US) are now rare and onlest is used mostly in African American Vernacular English (AAVE).  

The construct of the Old English ānlīc being ān (one) + -līc (-ly), only is thus understood in Modern English as on(e) + -ly.  One was from the Middle English oon, on, oan & an, from the Old English ān (one), from the Proto-West Germanic ain, from the Proto-Germanic ainaz (one), from the primitive Indo-European óynos (single, one).  It was cognate with the Scots ae, ane, wan & yin (one); the North Frisian ån (one), the Saterland Frisian aan (one), the West Frisian ien (one), the Dutch een & één (one), the German Low German een; the German ein & eins (one), the Swedish en (one), the Norwegian Nynorsk ein (one), the Icelandic einn (one), the Latin ūnus (one) & Old Latin oinos and the Russian оди́н (odín); doublet of Uno.

The –ly prefix was from the Middle English -ly, -li, -lik & -lich, from the Old English -līċ, from the Proto-West Germanic -līk, from the Proto-Germanic -līkaz (having the body or form of), from līką (body) (from whence Modern German gained lich); in form, it was probably influenced by the Old Norse -ligr (-ly) and was cognate with the Dutch -lijk, the German -lich and the Swedish -lig.  It was used (1) to form adjectives from nouns, the adjectives having the sense of "behaving like, having a likeness or having a nature typical of what is denoted by the noun" and (2) to form adjectives from nouns specifying time intervals, the adjectives having the sense of "occurring at such intervals".

The different phonological development of only and one was part of the evolution of English.  One was originally pronounced in the way which endures in only, atone and alone, a use which to this day persists in various dialectal forms (good 'un, young 'un, big 'un et al), the long standard pronunciation "wun" emerging around the fourteenth century in southwest and west England.  William Tyndale (circa 1494–1536), who grew up in Gloucester, used the spelling “won” in his translations of the Bible which were first published between 1525-1526 and the form slowly spread until it was more or less universal by the mid-eighteenth century.  The later use as indefinite pronoun was influenced by the unrelated French on and Latin homo.

Tyndale, before being strangled and burned at the stake in Vilvoorde (Filford near Brussels).  Woodcut from The Book of Martyrs (1563) by John Foxe (circa 1516-1587).

The cardinals and bishops in England probably neither much noticed nor cared about Tyndale’s phonological choice but they certainly objected to his choice of words in translation (church became “congregation” and priest became “elder”) which appeared to threaten both the institution of the Church and the centrality to Christianity of the clerical hierarchy.  Tried for heresy in 1536, he was pronounced guilty and condemned to be burned at the stake although, for reasons not documented, he was, after a ceremonial defrocking, strangled until dead while tied to the stake, his corpse then burned.

Activist herbivore Tash Peterson (b circa 1995, centre) at a vegan protest, Perth, Australia.

Although a thing which pedants enjoy correcting, the placement of “only” as a modifier matters only if putting it one place or the other would hinder clarity; there’s never been an absolute grammatical rule and, as long as the meaning is clear, it’s probably better to adopt whatever is the usual conversational style.  Strictly speaking, although “We only fuck vegans” means an assertion of a life consisting of nothing else, most would understand it as a statement of one who is prepared to contemplate intimacy only with vegans.  The best compromise to adopt is probably that recommended for handling the split infinitive: Use the more exact “We fuck only vegans” in formal use such as in writing and the more natural, conversational “We only fuck vegans” otherwise.  Note that a sign held aloft at a protest, although obviously something “in writing” is not an example of formal use; it’s just part of the conversation.

No ambiguity: Lindsay Lohan in sweatshirt from the I Only Speak LiLohan range.

Care must be taken to avoid ambiguity, especially in writing because the intonations of speech and other visual clues are not there to assist in the conveying of meaning.  Were one to say “She only fucks vegans after midnight”, quite what is meant isn’t clear and the sentence is better rendered either as “she fucks only vegans after midnight" (ie carnivores need not apply) or “she fucks vegans only after midnight” (ie vegans must wait till the midnight hour).  In informal English, only is a common sentence connector but again, this should be avoided in formal writing where “only” should be placed directly before the word or words that it modifies.