Word Origin & History
an erroneous word that, etymologically, means the exact opposite of what it is used to express, attested in non-standard writing from at least 1870s (e.g. "Portsmouth Times," Portsmouth, Ohio, U.S.A., April 11, 1874: "We supported the six successful candidates for Council in the face of a strong opposition.
We were led to do so because we believed every man of them would do his whole duty, irregardless of party, and the columns of this paper for one year has [sic] told what is needed."); probably a blend of irrespective and regardless. Perhaps inspired by the double negative used as an emphatic.