meaning of "Beam me up Scotty"

It comes from Star Trek (originally a TV series, then several movies). Scotty was the character in charge of transportation - instant teleportation from one place to another. They used to say 'beam me up Scotty' as an instruction to start teleporting them. So it basically means: get me out of here!

Move me from here by teleportation, through a beam:
http://en.wikipedia.org/wiki/Teleportation

It is correct than "Beam me up Scotty" was originally from "Star Trek" but that it meant "get me out of here" is far from the mark. The idiom is ironic. Teleportation is a futuristic fantasy and there is little theoretic evidence to actually support that it would work in practice.
The expression has come to apply to any idea that is "off the planet" i.e. impractical to the point of being loo

i disagree, "off the hook " would pertain to " impractical to the point of being loopy! "
beam me up scotty means get me out of here and we used the expresion as kids in the 70's/80's

Though it has become irrevocably associated with the series and movies, the exact phrase was never actually spoken in any Star Trek television episode or film.(Wikipedia)

Beam me up Scotty was never directly quoted in the films. The closest is "Scotty, beam me up".