お尻

Polite:

• bottom
• buttocks
• behind

Friendly (USA):

• butt
• buns

Vulgar:

• arse (UK)
• ass (USA)

Mistake:

• hip X「腰」

Simile meaning very smooth:

• As smooth as a baby's bottom.

Vulgar expressions meaning troublesome「面倒臭い」:

• I can't be arsed.
• It's a pain in the arse.

There are many more slang words.