A code-word for weed that can be used anywhere without anyone knowing what you're actually talking about. It's an evolution of the 1337 spelling for weed - 'w33d' The 33 was then taken from the middle of the word and is used on its own. Comes straight out of Caterham! Brap brap!
This has two meanings. Three more than Thirty or in Arabic 3ala 3airy which means in English "On my penis". It is not actually used for that purpose but it usually means that I don't care. The definition was created by two guys in high-school.