Origin and history of wacko

"crazy person," by 1938, probably a back-formation from wacky. As an adjective in slang sense of "worthless, stupid," it is attested from late 1990s.

colloquial or slang suffix, attested by 1880s in bucko, later (mostly 1910s and 20s) also in wino, ammo, combo, cheapo, kiddo, neato, boffo, the names of the Marx Brothers, commercial products (Sterno, Wham-O), etc. OED (1989) reports it "widespread in English-speaking countries but nowhere more so than in Australia."