Green’s Dictionary of Slang

(orig. US campus) used as a term of address, man.

1968		T. Wolfe Electric Kool-Aid Acid Test (1969) 71: Everybody is off the bus [...] laughing, giggling, yahooing, zonked to the skies on acid, because, mon, the woods are burning.
1977		Eble Campus Sl. Apr. 3: mon – an alternative pronunciation of man: Hey, mon, what you doing?
1992		J. Mowry Way Past Cool 18: Got us a little Rasta mon here. Peace, love, an ganja.