verb
quacks; quacked; quacking
[no obj] of a duck :to make a loud sound
noun
plural quacks
[count] :the loud sound that is made by a duck - compare 3quack
noun
plural quacks
[count] informal
disapproving :an unskillful doctor or a person who falsely claims to have medical skills
Brit - used in usually a negative or humorous way to refer to a doctor