힘숨찐 "Him Sum JJin" means a fool who hides his power, strength, and ability. 

In a positive sense, it refers to a loner who hides his strength. It refers to a person who looks like a fool on the outside but actually has great abilities.

힘 "Him" - power 

숨 "Sum" - hide 

찐 "JJin" - fool, 찐따. (Jjin Tda) 

*찐따 (Jjin Tda) is a word used to refer to someone who acts or thinks stupidly or foolishly and can also be used to refer to oneself. 

In a negative sense, it refers to a person who hides his strength against a  strong people and pretends to be weak, 

on the other hand, he (she) intentionally treats weak people more strongly. 

In this case, it is more common to express it as 강약약강 (Gang Yack Yack Gang) rather than 힘숨찐 "Him Sum JJin". 

강 (Gang) to powerful person rather than him(she)

약 (Yack) pretends to be weak

약 (Yack) to weak people

강 (Gang) pretends to more stronger than original himself(herself)