1706, initially a contraction of am perhaps not, plus in appropriate use with this feeling until it began to be properly used as a generic contraction for aren't, is not, etc., at the beginning of 19c. Cockney dialect of London; popularized by representations of the in Dickens, etc., which led to the word becoming banished from correct English.
A contraction for are not and am maybe not; in addition useful for isn't.u000du000a [Colloq. or illiterate speech]. See An't.
The Lakeman now patrolled the barricade, all the while keeping his eye on the Captain, and jerking out such sentences as these:--'It's not our fault; we didn't want it; I told him to take his hammer away; it was boy's business; he might have known me before this; I told him not to prick the buffalo; I believe I have broken a finger here against his cursed jaw; ain't those mincing knives down in the forecastle there, men? look to those handspikes, my hearties.