Green’s Dictionary of Slang

(US black) next to, up against; thus upside one’s head, of a blow, against the head.

1959		A. Anderson ‘The Checker Board’ in Lover Man 1: ‘You come on in this house before I hit you up side the head.
1965		H. Williamson Hustler 46: After I beat him up [...] the officer would hit him up side’ the head, and send him to the hole.
1969		C. Brown Mr Jive-Ass Nigger 60: Man, this dude was so mad with Jesus Christ that it was all he could do not to go up side Christ’s head.
1971		D. Goines Dopefiend (1991) 80: He slapped her upside the head.
1980		E. Folb Runnin’ Down Some Lines 259: upside 1. Alongside. 2. Up against.
1990		(con. c.1970) G. Hasford Phantom Blooper 33: Before he can get his shotgun out of the mud I lay my Tokarev 9-millimeter Russian officer’s pistol hard upside his head.
2002		W.D. Myers Handbook for Boys 36: If Cap was my age, I would have been upside his head and he knew it.