To curve or bend s.t. inward or upward; to make a face at s.o. with the mouth open and lips curved inward over the teeth (as a sign of disbelief, a sign that one didn't mean what he said, or to irritate s.o. [-an]).
The pa--un imp. focuses a bit more on the way you curve s.t.
Bend the mouth of that can in so its mouth will be small.
Maybe you didn't believe what he said because you are making a face at him (with your mouth open and lips curved inward over the teeth).