The word butt in the sense of buttocks was once considered unsuitable for general use. Comedians used it to get a laugh, but it was not considered acceptable in polite conversation. Children were taught to use less offensive colloquialisms like rear-end or backside. Nowadays the word has become so acceptable that it has largely replaced buttock and buttocks, even in formal contexts:
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Speakers used to attaching only one meaning to butt may be unfamiliar with other words that are spelled the same but have meanings unrelated to human or animal anatomy.
The Oxford English Dictionary has fourteen entries for the word butt as a noun.
Its sense of “human posterior” developed from this definition:
The thicker end of anything, especially of a tool or weapon, the part by which it is held or on which it rests; e.g. the lower end of a spear-shaft, whip-handle, fishing-rod, the broad end of the stock of a gun or pistol.
The etymology of butt in the sense of “thicker end” is obscure, but the word seems to be cognate with foreign words with such meanings as blunt, short, thickset, and stumpy.
This cut of meat does not come from the rear of an animal. It is the upper portion of a pork shoulder containing a small piece of the shoulder blade and characterized by leanness. The origin of this use of butt comes from this definition: “a cask for wine or ale; later, also a measure of capacity.”
On the upper east coast of colonial America, butchers packed less prized cuts of pork like the shoulder of the pig into butts (barrels) for storage and transport. The shoulder cut packed in this way became associated with New England, chiefly Boston, hence, “Boston butt.”
shooting an arrow at the butt
Another definition of butt is “a hillock (small hill) or a mound.”
A meaning that developed from butt as mound is “a mark for shooting.”
Archery targets were set up on a mound or embankment. Because there were usually two butts on an archery range, one at each extremity of the range, one might speak of “a pair of butts.” Another term for an archery range is “the butts.” This butt comes from French but: “goal, target.” This meaning of butt as target gives us the expression “to be the butt of a joke”: “an object at which ridicule, scorn, or abuse, is aimed.”
The word butt can also be used as a verb. One of its verbal meanings is “to strike, thrust, shove or push with the head or horns.” This butt is related to modern French bouter, “to strike, thrust, project” and gives us these expressions:
to butt in: to intrude where one is not wanted.
Example: Stop butting in our conversation.
to butt out: to stop intruding or interfering.
Example: I told him to butt out of my private affairs.
A curious fact about the shortening of buttock to the shorter form butt is that buttock is itself a diminutive form of butt (“thicker end of something”). The suffix -ock is the same one that makes hillock mean “a small hill.”