'Butt' and 'ass' are both American terms referring to the same body part. Both are very informal, but 'ass' is the more vulgar of the two words. You wouldn't use either word in polite situations. 'Buttocks' is neutral, and could be used in any situation, informal or formal. Strictly speaking, it refers just to the two cheeks that you sit on, not to the ...um... functional part of that area of your anatomy. We rarely use the word 'buttocks', though, because we rarely need to refer to the cheeks specifically. A physiotherapist might use it, for example, when explaining exercises to their patient. It certainly isn't a common everyday word. NB A more useful term is 'bottom', which is not considered vulgar. You could use the word 'bottom' when talking to a child or a stranger without causing offence. 'Hip' refers to a different body part entirely. Your hip is the SIDE of your body between your waist and your legs. Someone armed with a gun would wear it on their hip, or a mother might carry a baby on her hip. Your leg bones are connected to your body via your hip bones.

If I was talking to my Nan, I'd say "bottom". If I was talking to my Mum, I'd say "bum". If I was talking to children in my own family or my friend's children, I'd say "bum". If I was talking to people my own age, I'd say "bum" or "arse". (I'm a 27-year-old woman.) I'd never say "butt" because I'm British, not American. I'd also never say "buttocks", and would laugh if someone said that to me. Hips are the curve from waist to the top of the bum. For me, wriggling my hips (like in salsa dancing) is completely different to wriggling my bum (which I picture like twerking).

(U.S. English) It is somewhat rude to refer to the buttocks at all. In a foreign language I would try to avoid talking about them unless really necessary. To a doctor, say "buttocks." To friends that you can joke with, say "butt." "Buttocks" is standard English. It's neither formal nor informal. It's a term used with animals. It's a term used in plain factual descriptions. It is used more in writing than in speaking. "When you are sitting in a chair with proper posture, your weight will be equal on each of your buttocks." "When I was a child I slid on a wooden floor and got a splinter in my left buttock." "Butt" has become the most common and acceptable word in everyday spoken U.S. speech. It's informal. "Ass" is a "bad word." It's not very bad, but you would never use it in a polite or formal situation. It is ambiguous as to whether it means the buttocks, the anus, or the entire area down there. It is written and pronounced the same way as the word that means a donkey (horse-like animal) but it's a completely different word. A "hip" is a slightly different part of the body. It's easiest to understand if you look at a picture of a skeleton and see where the hip bone is and what its shape is. "Buttocks" are the fleshy muscles, "hips" are the bones. You sit on your buttocks. Your legs are socketed into your hips. "Hip" can also mean the ball-shaped part of the leg bone; when you read that someone has "broken their hip" or "has had a hip replacement" it means part of the leg bone, not the hip bone. "Hip" is completely acceptable and polite but it can't usually be used to mean the same thing. However, when someone says that a woman "wiggles her hips" it is a polite way of saying "wiggles her butt," and in fashion the "hip measurement" is the measurement all the around the part of the body that includes both the hip bones and the buttocks. The buttocks muscles are the "gluteus maximus" and athletes call them the "glutes."