What does the letters lmb stand for?

Commonly known as "LMB". It is a term generally used to taunt or insult others. Example: Hey I just beat you in a foot race, so you can lick my butthole! It is also commonly used when chatting on facebook, a person will send someone "LMB" and quickly log off of their account. This makes it so the receiver of the message can not respond. However, some people actually mean that they want their anus licked when they use the phrase.