late 12c., buhsum "humble, obedient," from Old English bugen "to bow" (from Proto-Germanic *bugan-, from PIE root *bheug- "to bend") + -som (see -some (1)), for a total meaning "capable of being bent." Related: Buxomly; buxomness.
The meaning progressed from "compliant, obliging," through "lively, jolly," "healthily plump, vigorous and attractive," to (in women, and perhaps influenced by lusty) "attractively plump, comely" (1580s). In Johnson  the primary meaning still is "obedient, obsequious." It was used especially of women's figures from at least 1870s, and by 1950s it had begun to be used more narrowly for "bosomy" and could be paired with slim (adj.). Among its cognates are Dutch buigzaam, German biegsam "flexible, pliable," which hew closer to the original English sense.