butt (n.1)

"thick end," c. 1400, butte, which probably is related to Middle Dutch and Dutch bot, Low German butt "blunt, dull," Old Norse bauta, from Proto-Germanic *buttan, from PIE root *bhau- "to strike." Or related somehow to Old English buttuc "end, small piece of land," and Old Norse butr "short," from Proto-Germanic *butaz, which is from the same PIE root. Also probably mixed with Old French bot "extremity, end," which also is from Germanic (compare butt (n.3)). The meaning "remainder of a smoked cigarette" is recorded by 1847.

"liquor barrel, cask for wine or ale," late 14c., from Anglo-French but and Old French bot "barrel, wine-skin" (14c., Modern French botte), from Late Latin buttis "cask" (see bottle (n.)). It is cognate with Spanish and Portuguese bota, Italian botte. Usually a cask holding 108 to 140 gallons, or roughly two hogsheads; at one time a butt was a legal measure, but it varied greatly and the subject is a complicated one (see notes in Century Dictionary).

"target of a joke, object of ridicule," 1610s, from earlier sense "target for shooting practice, turf-covered mound against which an archery target was set," (mid-14c.), from Old French but "aim, goal, end, target" of an arrow, etc. (13c.), which seems to be a fusion of Old French words for "end" (bot) and "aim, goal" (but), both ultimately from Germanic. The latter is from Frankish *but "stump, stock, block," or some other Germanic source (compare Old Norse butr "log of wood"), which would connect it with butt (n.1).

"hit with the head, strike by thrusting" (as with the end of a beam or thick stick), c. 1200, from Anglo-French buter, Old French boter "push, shove, knock; thrust against," from Frankish or another Germanic source (compare Old Norse bauta, Low German boten "to strike, beat"), from Proto-Germanic *butan, from PIE root *bhau- "to strike."

The meaning "join at the end, be contiguous" is from 1660s, partly a shortening of abut. To butt in "rudely intrude" is American English slang, attested from 1900. Related: Butted; butting.

"flat fish," c. 1300, a general Germanic name applied to various kinds of flat fishes (Old Swedish but "flatfish," German Butte, Dutch bot), from Proto-Germanic *butt-, name for a flat fish, from PIE root *bhau- "to strike." "Hence butt-woman, who sells these, a fish-wife." [OED]

"a push or thrust with the head," 1640s, from butt (v.).

"posterior, buttocks, rump," from mid-15c. in cookery, in reference to animal parts, probably from or related to butt (n.1) "thick end," or short for buttock. In modern use chiefly of humans, probably an independent derivation, attested by c. 1860 in U.S. slang.