say someone is ass,what does it mean?

Question

Answer

An ass (or donkey), 驴子,  is a barn animal which is quite stupid. Calling someone an ass is like calling him very stupid!

Answer

Depends on the context in which it is used:

ass = 讨厌
You are an ass! (你很讨厌！)

pain in the ass = 难缠
You are such a pain in the ass! (你很难缠！)

dumb ass = 笨蛋

Answer

I take it as depending on if there is a particle or not.

For "an ass", you are saying someone is as stupid and stubborn as a donkey.

For simply "someone is ass", you're saying that they are as pleasing and appealing as their own behind.  In both contexts it's a negative meaning (imagine the ass/arse is fat, hairy and putrid).