Maybe Yes, Maybe No.
Depends who you ask.
The Merriam Webster Dictionary says “Yes.”
Yes, “Irregardless” Is a Real Word, a Nonstandard Word
Typing “irregardless” into a Word document generates a red squiggly line beneath it that signifies something is wrong; perhaps the word is misspelled or it’s not a word at all. However, Merriam-Webster Dictionary says this North American colloquialism is, in fact, a word, just one that is “nonstandard.”
“The most frequently repeated remark about it is that ‘there is no such word,’” the dictionary entry explains. “There is such a word, however.”
All the Words Fit To Print
The question of whether “irregardless” is really a word has received more attention in recent years, but the legitimacy of the word was established in the early 20th century. The Oxford English Dictionary cites its appearance in the 1912 edition of the American Dialect Dictionary, but its origins go even further back, with the possible earliest printed appearance of the word found in 1795. A poem called “The Old Woman and Her Tabby,” printed in the City Gazette & Daily Advertiser of Charleston, South Carolina, included the lines: “But death, irregardless of tenderest ties / Resolv’d the good Betty, at length, to bereave.”
“The Old Woman and Her Tabby” aside, most style guides (manuals that dictate the grammar and writing standards for publications and formatting documents) eschew “irregardless.” The AP Stylebook says, “A double negative. Regardless is correct.” Chicago Manual of Style editors state, “At the moment … there’s no reason to change a perfectly good word like ‘regardless’ to one that is bound to raise the hackles of many readers.”
The widespread recognition of “irregardless” by dictionaries, however, simply indicates the word continues to be used with a specific and consistent meaning. Or, as the editors at Merriam-Webster so succinctly explain it, “We do not make the English language, we merely record it.”
But why all the fuss over this word in the first place?
Two Negatives Don’t Make a Positive
Perhaps the conflict comes from the appearance that “irregardless” and “regardless” should be opposites, but are actually synonyms. But “irregardless” seems to receive more criticism than similar words, such as “inflammable,” which means the same as “flammable” (“capable of being set on fire”), and “deboned,” which has the same definition as “boned” (“having had the bones removed from”). The word “irregardless” seems to draw such ire because it has both a negative prefix (ir-) and a negative suffix (-less.)
Grammarians and English teachers might argue that “irregardless” isn’t a real word because it doesn’t make sense. If “regardless” means “without regard,” then “irregardless,” with its negative prefix and suffix, is a double negative that means “without without regard.” Some lexicographers, however, suggest that the issue with “irregardless” is less about what the parts literally mean and more about its perception. Words such as “mines,” “ain’t,” and “irregardless” are held up by some as a marker of education and not necessarily class or manners.
While grammar gatekeepers apply a rigid set of rules to the English language, linguists consider it a living thing that evolves and grows. The etymology of “irregardless,” according to the OED, is “probably a blend of irrespective and regardless.” Note the “probably.”
In biological evolution, random changes are known as “drift,” and linguists have applied the same concept to the English language, positing that drift has played a significant part in the evolution of the language.
Explaining how language evolves is not an exact science — sometimes the best a linguist can do is “probably.” In the meantime, “irregardless” continues to be a real word that has been around for a very long time, irregardless of what your English teacher may have told you.
CMOS says “NO.”
Irregardless is Not a Word. Use regardless (or possibly irrespective).
5.250: Good usage versus common usage
The best dictionaries are signaled by the imprints of Merriam-Webster, Webster’s New World, American Heritage, Oxford University Press, and Random House. But one must use care and judgment in consulting any dictionary. The mere presence of a word in the dictionary’s pages does not mean that the word is in all respects fit for print as Standard Written English. The dictionary merely describes how speakers of English have used the language; despite occasional usage notes, lexicographers generally disclaim any intent to guide writers and editors on the thorny points of English usage—apart from collecting evidence of what others do. So infer is recorded as meaning, in one of its senses, imply; irregardless as meaning regardless; restauranteur as meaning restaurateur; and on and on. That is why, in the publishing world, it is generally necessary to consult a style or usage guide.