"'Bastard', meaning 'one begotten and born out of wedlock; an illegitmate or natural child' comes from the Old French bastard = 'fils de bast,' 'pack-saddle child,' f. 'bast' (see BAST + the pejorative suffix -ARD. C. BANTLING.

BAST is from OFr bast, medieval Latin bastum, 'pack-saddle' (used as a bed by muleteers in the inns) in phrase 'fils (homme, etc.) de bast,' literally 'pack-saddle child,' as opposed to a child of the marriage-bed, thus forming a tersely allusive epithet for illegitimate offspring.

-ARD is suffix...OFr -ard, -art, German -hart, -hard, 'hardy,' often forming part of personal names as in OHG Regin-hart (Raynard)...Used in Fr as masculine formative...often pejorative...It appeared in Middle English in words from OFr, as bastard, coward, mallard, wizard...and became at last a living formative of English derivatives, as in buzzard, drunkard, laggard, sluggard, with the sense of 'one who does to excess, or who does what is discreditable.

BANTLING is possibly from BAND, swathe + -LING, but considered by Mahn, with greater probability, a corruption of Ger 'bankling' bastard from 'bank' bench, i.e. 'a child begotten on the bench, and not in the marriage bed.'...used formerly as a synonym of 'bastard.'. "