Origin and history of -o

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."

The popular U.S. newspaper comic strip Knocko the Monk (monkey) by Gus Mager debuted in 1904, The names of Knocko's monkey foils in the strip generally kept the -o (Kiddo, Braggo, Tightwaddo; there is a character Groucho by 1907).