1.
A fictional astronaut in David Bowie's lyrics. Also, as a metaphor: alteredstate of consciousness (with or without drugs), people in such state, who lose a sense of reality (have no ground control).
My mama said to get things done you'd better not mess with Major Tom (Ashes to Ashes by D.Bowie).
See david bowie, drugs, bowie, ground control, ziggy stardust, rock music
2.
major tom is a heroin addict
that guy is a major tom
See horse with no name, smack, junk, heroin, dope
3.
A bonerspecific to the artist David Bowie. Once it goes up, it generally will never come back down.
I just saw a live recording of David Bowie and now I have a huge Major Tom.
See boner, david bowie, ziggy stardust, rock