From Word and Phrase: True and False Use in English by Joseph Fitzgerald (Chicago: A. C. McClurg & Co., 1901), page 255:

Buxome is the exact equivalent, etymologically, of German beugsam, which means literally, flexible, pliable, pliant, being from the verb beugen, to bend; in German beugsam is hardly used but in this literal sense. Buxom or buxome is of the same origin as beugsam, and is made up of the same two elements: literally, it means what beugsam means, flexible, pliant; but it early assumed a figurative sense,—that of compliant, yielding; complaisant, obedient, and the like, as "to make thee buxome to her law." Of course in that sense a colt might be subdued to buxomeness; or a henpecked man might be buxome to his wife—obedient to her. But whatever was written in the old times was written by the men, and buxome, under their pen, assumed the character of a feminine adjective,—one expressive of distinctively womanly virtues, the chiefest of which is of course that of complaisance toward her lord; but even that meaning was in time lost, and the epithet came to signify plumpness, freshness of complexion, and abundant, ebullient animal spirits, with at least a suspicion of something else and more; the "buxome lass" was no favorite of the matron. Spenser, as was to be expected, retains much of the ancient signification of the word:

"And bene of ravenous wolves yrent,
All for they would be buxome and bent."

So too when in prose he speaks of the people being made "buxome to government." Chaucer gives a good illustration of the use of Buxome in the best sense, as denoting the behavior of a faithful and loving wife:

"For who can be so buxom as a wyf?
Who is so true and eek so ententyf?"

And the hermit nun of Norwich, Chaucer's contemporary (she was still living 1388, aged forty-five years), has this: "We be all mercifully beclosed in the mildhed of God and in his meekhed, in his benignity, and in his buxomness." And in another place: "When a soul is tempted ... it is time to praie to make herself supple and buxom to God.