A conversational retort born in the 80's (Via UK kids show Blue Peter in conjuciotn with spastic Joey Decon)
The "belm craze" spread quickly between all children from 5-55.
The Phrase is pronounced towards one who you are mocking or generally disagreeing with in conversation.
The word "belm" is pronounced by inserting ones tongue between the lower front teeth and lower front lip and pronouncing the word "belm". The "lm" aspect of the word can be "held" for many seconds for great effect; EG: Belllllllllllmmmm". The pronunciation can vary in tone depending on how "spazy" one wants to make the respondent think of the power of the "belm". EG: One could choose to "beeeeeeeeellllllllllllllllllllmmm" at the respodent via the above proper method.
For added effect to a belm it is normal to rub ones hand up and down along the enlarged front lip area or to slap it in case of greater insult to be conveyed.
One on one contact: One could also choose to do the belm but only utter the sound "mmm" with the tongue stuck in belm position, this is a subtle but thorough belm and should not be disregarded as a poor belm, it is damning. This is the ultimate close contact personal insult. Don't do it unless you really dislike them.