Originally used by schoolkids in the 1970's, to express your opinion that the person you are speaking to is not telling the truth.
Always pronounced with your tongue stuck between your bottom lip and bottom teeth, so as to push out your bottom lip more than normal.
Often supplemented by holding said bottom lip area with your thumb and index finger, as if you were pulling your lip out.
Silly voice can be added for extra effect.
"You've got a space -invader machine in your bedroom? Belm!"
"He's such a belmer"
"You sh*gged a girl in fifth year? BELM! BELM! BELM!"
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