barrelful (n.)

"cylindrical vessel or cask, generally bulging in the middle and made of wooden staves bound by hoops," c. 1300, from Old French baril "barrel, cask, vat" (12c.), with cognates in all Romance languages (Italian barile, Spanish barril, etc.), but of unknown origin. Also a measure of capacity of varying quantity.

The meaning "metal tube of a gun" is from 1640s. Barrel-roll (n.) in aeronautics is from 1920. To be over a barrel figuratively, "in a helpless or vulnerable condition," is by 1914 and might suggest corporal punishment.

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.).