Meaning of 'UN in English

'UN

'un / ə n/ BrE AmE pronoun British English spoken

good 'un/bad 'un/little 'un etc a short form of ‘one’, used to say that someone or something is good, bad etc. Teachers and careful speakers of English do not use this expression:

He’s a bad 'un.

I ought to collect the little 'uns (=young children) from school.

Longman Dictionary of Contemporary English.      Longman - Словарь современного английского языка.