/bʌt/
[noun] A word with several meanings: most commonly, the thick or bottom end of something (like a rifle), a large barrel used to store wine or water, a target for archery practice, or an informal word for the human bottom.
Before it meant your backside, a 'butt' was an archery target — which is why we still say someone is 'the butt of a joke'.
The word 'butt' is really four or five different words that collided over centuries, each arriving from a different direction and gradually blurring together into one wonderfully versatile English term. The most historically important of these is the archery sense. This comes from Old French 'but', meaning a goal or target, which is itself related to Old French 'bouter' — to push or strike — giving us also the modern English verb 'to butt' as in 'to butt heads'. The archery butt was a mound of earth or a flat board set up as a target, and ranges where archers practised were called 'the butts'. This sense survives perfectly in the phrase 'the butt of a joke', meaning the person or thing that everyone aims their laughter at.
The barrel sense comes from a completely different root. Here, 'butt' derives from Old French 'botte' and Medieval Latin 'butta', both meaning a cask or large vessel for liquid. This Latin term likely has roots in Late Latin 'buttis', also a cask, though its ultimate ancestry is uncertain. A butt in this sense was a specific unit of measurement — roughly 108 gallons of wine or ale — and the word was in regular use in English by the 14th century for large storage barrels. You can still find 'butt' used in this sense in historical documents about wine cellars and royal household accounts.
The sense of 'thick end' — as in the butt of a rifle or a cigarette butt — appears to come from a Germanic root related to the Low German 'butt' and Middle Dutch 'bot', both meaning blunt or stumpy. Old Norse 'bútr', meaning a short log, is closely related. This cluster of words gave English the idea of something that is cut off, blunt, or forms a solid end. From here we get 'buttock', which follows naturally — the rounded, blunt end of the human body. The suffix '-ock' is a common Old English diminutive, so 'buttock' literally means 'little butt' in this blunt-end sense.
The informal American English use of 'butt' to mean the human bottom or backside rose to full popularity in the 20th century, though it was in colloquial use well before that. It appears to have developed from 'buttock' through simple shortening, aided by the existing sense of a blunt, rounded end. British English generally preferred 'bottom' or 'bum' in polite use, while American English enthusiastically adopted 'butt', which today sounds far more casual than crude in everyday American speech.
What makes 'butt' so fascinating is that these different streams — the French archery target, the medieval wine barrel, the Germanic blunt end — all flowed into the same short word without any of them being related to the others at their origins. English speakers simply used the same sound for very different things, and over time the meanings bled into one another. The cigarette butt, the rifle butt, the water butt in a garden, the butt of office jokes, and the act of butting in on a conversation are all, strictly speaking, different words wearing the same clothes — a perfect example of how English accumulates meaning through happy accident as much as through deliberate design.