(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!
A shortened form of the phrase: "Must be nice". Created out of pure laziness of not wanting to say the entire phrase. Sometimes the word "It" is placed in front, meaning: "It must be nice".