bastard

From Middle English bastard, bastarde, from Old English bastard (used as an epithet), from Anglo-Norman bastard, Old French bastart (“illegitimate child”), perhaps via Medieval Latin bastardus, of obscure origin.

Likely from Frankish *bāst (“marriage, relationship”) + Old French -ard, -art (pejorative suffix denoting a specific quality or condition). Frankish *bāst derives from a North Sea Germanic variety of Proto-Germanic *banstuz (“bond, connection, relationship, marriage with a second woman of lower status”), from Proto-Indo-European *bʰendʰ- (“to tie, bind”) and is related to West Frisian boaste (“marriage, matrimony”), Middle Dutch bast (“lust, heat”), and more distantly to English boose (“cow-stall”). The term probably originally referred to a child from a polygynous marriage of heathen Germanic custom, a practice not sanctioned by the Christian churches.

Alternatively, and less likely, Old French bastart may have originated from the Old French term fils de bast (“packsaddle son”), meaning a child conceived on an improvised bed (medieval saddles often doubled as beds while travelling). However chronology makes this difficult, as bastard is attested in Old French from 1089 (Middle Latin bastardus as early as 1010), yet Old French bast (modern French bât), though attested since 1130 with the meaning of "beast of burden", doesn't acquire the specific meaning of "packsaddle" until the 13c., making it too late to have given rise to the terms bastard and bastardus with this sense. The French Centre National de Ressources Textuelles et Lexicales supports the Germanic theory further above as being most likely.