Respect bro!! , you never hear anything like ''Fear, man'' or ''honesty, man.'' used in the same sense, its interesting.
The Green’s Dictionary of Slang suggests that the expression originated in the black communities and is from the ‘90s:
(W.I./UK/US black teen) a phr. used to accord the subject the respect they have earned on the basis of earlier positive or praiseworthy actions; also used as a greeting.
1991 [UK] Reeves & Mortimer Vic Reeves Big Night Out n.p.: Yo! Respect, Dennis!
1997 [UK] C. Newland Scholar 46: Respec’ cuz, for that ironin’ lick.