sickoU.S.slang.|ˈsɪkəʊ| [f. sick a. + -o2.] = sickie 2. 1977J. Wambaugh Black Marble (1978) xi. 245 But, Clarence, listen! She's a sicko. Some kinda fruitcake or somethin.1982Chicago Sun-Times 25 Nov. 7/2 Is it asking too much for these sickos to stop bothering decent women?