This phrase means to give someone a light reprimand or warning for their actions or behavior.  توبیخ کردن یا تذکر دادن سبک و ملایم برای اعمال یا رفتار شخصی 
مثال؛
The teacher rapped the student on the knuckles for talking in class.  
 ...  [مشاهده متن کامل]  
A supervisor might rap an employee on the knuckles for not following the company’s guidelines. 
In a sports game, a referee might rap a player on the knuckles for unsportsmanlike conduct.