Meaning of 'UN in English

'UN

/ ən; NAmE / pronoun

( BrE , informal ) a way of saying or writing 'one' :

That was a good 'un.

The little 'uns (= the small children) couldn't keep up.

Oxford Advanced Learner's English Dictionary.      Оксфордский английский словарь для изучающик язык на продвинутом уровне.