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." Figurative sense of "something not pure or genuine" is late 14c. 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.
1520s, the earliest recorded sense is "mistress;" the allusion to grass is not clear, but it commonly was believed to refer to casual bedding (compare bastard and German Strohwitwe, literally "straw-widow," and compare the expression give (a woman) a grass gown "roll her playfully on the grass" (1580s), also euphemistic for the loss of virginity). Revived late 18c. as "one that pretends to have been married, but never was, yet has children;" in early 19c. use it could mean "married woman whose husband is absent" (and often presumed, but not certainly known to be, dead), also often applied to a divorced or discarded wife or an unmarried woman who has had a child. Both euphemistic and suggestive.
[G]rasse wydowes ... be yet as seuerall as a barbours chayre and neuer take but one at onys. [More, 1528]
GRASS WIDOW, s. a forsaken fair one, whose nuptials, not celebrated in a church, were consummated, in all pastoral simplicity, on the green turf. [Rev. Robert Forby, "Vocabulary of East Anglia," London, 1830]
"contemptible person, moocher," 1560s, from Hebrew, literally "bastard" (used as such in Vulgate), but the modern word (by 1914) is a separate borrowing from Yiddish.
"bastard," 1560s, from Late Latin mamzer, from Hebrew mamzer, left untranslated in Deuteronomy xxiii.2 in the Vulgate. Modern uses (from mid-20c.) probably are from Yiddish.