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late 12c., buhsum "humble, obedient," from Proto-Germanic *buh- stem of Old English bugen "to bow" (see bow (v.)) + -som, for a total meaning "capable of being bent."
Meaning progressed from "compliant, obliging," through "lively, jolly," "healthily plump, vigorous," to (in women, and perhaps influenced by lusty) "plump, comely" (1580s). Used often of breasts, and by 1950s it had begun to be used more narrowly for "bosomy" and could be paired with slim (adj.). Dutch buigzaam, German biegsam "flexible, pliable" hew closer to the original sense of the English cognate.