Irregardless is a word commonly used in place of regardless or irrespective, which has caused controversy since the early twentieth century, though the word appeared in print as early as 1795. Most dictionaries list it as "nonstandard" or "incorrect."

The origin of irregardless is not known for certain, but the speculation among references is that it may be a blend, or portmanteau word, of irrespective and regardless, both of which are commonly accepted standard English words. By blending these words, a word is created whose meaning is not predictable from the meanings of its constituent morphemes. Since the prefix ir- means "not" (as it does with irrespective), and the suffix -less means "without", the word irregardless could therefore be expected to have the meaning "in regard to", instead of being merely a synonym of regardless. According to the Oxford English Dictionary (OED), Irregardless was first acknowledged in 1912 by the Wentworth American Dialect Dictionary as originating from western Indiana,] though the word was in use in South Carolina before Indiana became a territory. The usage dispute over irregardless was such that, in 1923, Literary Digest published an article titled "Is There Such a Word as Irregardless in the English Language?" The OED goes on to explain the word is primarily a North American colloquialism.

The approach taken by lexicographers when documenting a word's uses and limitations can be prescriptive or descriptive. The method used with irregardless is overwhelmingly prescriptive. Much of the criticism comes from the double negative pairing of the prefix (ir-) and suffix (-less), which stands in contrast to the negative polarity exhibited by most standard varieties of English. Critics also use the argument that irregardless is not, or should not be, a word at all because it lacks the antecedents of a "bona fide nonstandard word." A counterexample is provided in ain't, which has an "ancient genealogy," at which scholars have not leveled such criticisms.

Australian linguist Pam Peters (The Cambridge Guide to English Usage, 2004) suggests that irregardless has become fetishized because natural examples of this word in corpora of written and spoken English are greatly outnumbered by examples where it is in fact only cited as being incorrect.