Green’s Dictionary of Slang

(US black) to behave in an arrogant, boastful manner, to show off.

1965		E. Cleaver Soul on Ice 38: High-siding. Cutting up; having fun at the expense of others.
1967		M. Braly On the Yard (1977) 210: So call yourself lucky and knock off the highsiding.
1972		Milner & Milner Black Players 252: What do I care to highside and ride down Fillmore Street in my Cadillac for? That’s bein’ a trick, you dig?
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Highside (verb) To show off; to flaunt; to act conceited.