不三不四 is a four-character idiom (成语) used to describe someone of dubious character or behavior that is neither proper nor acceptable. Literally 'not three, not four', it originates from the ancient numbering system where 三 (three) and 四 (four) were associated with proper conduct; falling outside both means being improper or shady. You can use it to criticize someone's conduct, especially when they don't act according to social norms.