(1) When something is just plain weird, stupid, or fake.
(2) When someone says something weird, stupid, or a bad comeback.
(3) Complete rejection
(1) Movie: A man falls off a 10 story building and keeps running. Background -Muh!
(2) Statement: Man, your mom is like a shoe, she keeps walking all over the place. Background: -MUH!
(3) Q: Hey, Do you want to go bowling? Ans:-MUH!