boast: [13] The immediate source of boast appears to be Anglo-Norman bost, but where it came from before that is far from clear; German dialect bauste(r)n ‘swell’ has been compared, suggesting that it could be of Germanic origin. To begin with it meant ‘loud or threatening talk’ as well as ‘bragging’.
boast (n.)
mid-13c., "arrogance, presumption, pride, vanity;" c. 1300, "a brag, boastful speech," from Anglo-French bost "ostentation," probably via Scandinavian (compare Norwegian baus "proud, bold, daring"), from Proto-Germanic *bausia "to blow up, puff up, swell" (cognates: Middle High German bus "swelling," dialectal German baustern "to swell;" Middle Dutch bose, Dutch boos "evil, wicked, angry," Old High German bosi "worthless, slanderous," German böse "evil, bad, angry"), from PIE *bhou-, variant of root *beu-, *bheu- "to grow, swell" (see bull (n.2)).
The notion apparently is of being "puffed up" with pride; compare Old English belgan "to become angry, offend, provoke," belg "anger, arrogance," from the same root as bellows and belly (n.). Related: Boasted; boasting. An Old English word for "boasting" was micelsprecende, "big talk."
boast (v.)
early 14c., "to brag, speak arrogantly;" from the same source as boast (n.). Related: Boasted; boasting.