vô năng 
incapable, unfit, incompetent [ vô ] : (1) in-, im-, (negative prefix); (2) in, at, on; (3) to go into, enter [= vào]
[ năng ] : ability; often, frequently, many a time
Most Common Compounds Set 1:
Most Common Compounds Set 2 :
vô năng incapable, unfit, incompetent