good 'un/bad 'un/little 'un etc spokena shortform of ‘one’, used to say that someone or something is good, bad etc. Teachers and careful speakers of English do not use this expression 一个好的/坏的/小的等人[东西]〔one 的缩略式;教师以及说英语讲究的人不用这种说法〕
He’s a bad 'un.
他是个坏家伙。
I ought to collect the little 'uns (=young children) from school.