boastful (adj.)

mid-13c., "arrogance, presumption, pride, vanity;" c. 1300, "a brag, boastful speech," from Anglo-French bost "ostentation," probably from a Scandinavian source (compare Norwegian baus "proud, bold, daring"), from Proto-Germanic *bausia "to blow up, puff up, swell" (source also of 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-, a root supposed to have formed words associated with swelling (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.). The meaning "a cause of boasting, occasion of pride" is from 1590s. Related: Boasted; boasting. An Old English word for "boasting" was micelsprecende, literally "big talk."

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).