1. [late 19C+] (US) a man or boy; thus main guy under main adj.
2. [1910s+] a general term of address, repopularized in 1970s+ among young UK blacks, and now in general teen use.
3. [late 19C+] (US) a woman.
4. [1960s] a boyfriend, a lover.
5. [1970s+] a person, irrespective of gender.
6. [1980s+] (US campus) in ironic reversal, an incompetent, an inadequate.
7. [1980s+] (US) an object, a thing.