"illegitimate child," early 13c., from Old French bastard "acknowledged child of a nobleman by a woman other than his wife" (11c., Modern French bâtard), probably from fils de bast "packsaddle son," meaning a child conceived on an improvised bed (saddles often doubled as beds while traveling), with pejorative ending -art (see -ard). An alternative possibly is that the word is from Proto-Germanic *banstiz "barn," equally suggestive of low origin.
Compare German bänkling "bastard; child begotten on a bench" (and not in a marriage bed), the source of English bantling (1590s) "brat, small child." Bastard was not always regarded as a stigma; the Conqueror is referred to in state documents as "William the Bastard."
The figurative sense of "thing not pure or genuine" is by late 14c. Its use as a generic vulgar term of abuse for a man is attested from 1830. Among the "bastard" words in Halliwell-Phillipps' "Dictionary of Archaic and Provincial Words" are avetrol, chance-bairn, by-blow, harecoppe, horcop, and gimbo ("a bastard's bastard").
As an adjective from late 14c. It is used of things spurious or not genuine, having the appearance of being genuine, of abnormal or irregular shape or size, and of mongrels or mixed breeds.