ain't       
not standard uk/eɪnt/ us/eɪnt/
 am not、is not、are not、has not或have not的缩略形式
short form of am not, is not, are not, has not, or have not
  He ain't going. 他不会去了。 
 "Can I have a cigarette?" "I ain't got none left." “能给我支烟吗?”“我都抽完了。” 
 - More examples
 - As my old ma used to say, you can't spend what you ain't got.
 - So he's a bit of an idiot, but he's still my bruv, ain't he?
 - I ain't done nothing wrong.
 - Lend me a fiver - I ain't got no money right now.
 - That sounds awful - Sinatra he ain't!
 
Idiom  
if it ain't broke, don't fix itidiom  informal saying 不需修,别多手;不破就不修
said when you recognize that something is in a satisfactory state, and there is no reason to try to change it