Middle English word bac- comes from Old English an (A; an (indefinite article) Lone. Sole (cardinal) one.), Old English bæc

Detailed word origin of bac-

Dictionary entry	Language	Definition
an	Old English (ang)	A; an (indefinite article) Lone. Sole (cardinal) one.
bæc	Old English (ang)	Stream; brook (anatomy) back. Back or rear end (of something).
aback	Middle English (enm)
bak-	Middle English (enm)

Words with the same origin as bac-