Irregardless

Irregardless (pronounced ir-i-gahrd-lis)

A historically nonstandard adverb which means “regardless”.  Some descriptive dictionaries have accepted it as a word; prescriptive volumes have not.

Circa 1870s: Thought probably a portmanteau of irrespective +‎ regardless, the word can also be analysed as ir- (from the Latin prefix -ir, an assimilated form of in- (used before r-) used for expressing negation; not) +‎ regardless because it may be the prefix ir- was added to amplify the negative in regardless, as plain negatives did at the time the word came into use (and continue still to do in dialects).

The fetish

Historically, irregardless has been thought non-standard because of the two negative elements ir- and –less, most authorities suggesting it was probably formed on the analogy of such words as irrespective, irrelevant, and irreparable.  Technically, it’s an erroneous word which etymologically, means the opposite of what it is used to express; it means “regardless” so is entirely unnecessary.  However, many dictionaries include a definition while noting it’s a non-standard form, the more rigorous insisting it’s incorrect, noting the controversy since the early twentieth century and suggesting "regardless" should instead be used.  So it's a a nonsensical word, as the ir- prefix usually functions to indicate negation and the only possible case to be made is the ir- could operate as an intensifier; few feel moved to make the case.  Similar ir- words, while rare, do exist in English, including irremediless ("remediless"), irresistless ("resistless") and irrelentlessly ("relentlessly”) but irregardless remains incorrect, even sympathetic dictionaries noting they record an evolving language as it is spoken, not as advocates for adoption of non-standard forms.

The Collins Dictionary finds little to suggest use has ever been anything but rare.

The Oxford English Dictionary (OED) cites a 1912 entry in the Wentworth American Dialect Dictionary as the first instance of official record, the entry suggesting an origin in western Indiana though the it seems the word was in use in South Carolina before Indiana became a territory.  Disputes were noted as early as 1923 and it was long regarded as an oddity, a North American colloquialism and, being a recent one, subject to more scholarly criticism than a word like “ain’t” which enjoys an ancient genealogy.  Some publications suggest its only use is as a class-identifier or educational marker.  That may be of limited utility because there’s little evidence the word is in even rare use and it seems editors of dictionaries feel compelled to include an entry either to (1) express disapprobation or (2) note that regardless of the etymological rights and wrongs, decades of use justify acknowledgement.  It’s thus become something of a lexicographical fetish; natural examples of this word in the corpora of written and spoken English being overwhelmingly outnumbered by instances where it appears only for the purpose of being condemned as incorrect.

It is though a perfect Mean Girls (2004) word.  Gretchen Wieners (Lacey Chabert (b 1982)) illustrates.