cocksucker (n.)

1890s, "one who does fellatio" (especially a male homosexual); 1920s as "contemptible person," American English, from cock (n.1) in phallic sense + sucker (n.). Used curiously for aggressively obnoxious men; the ancients would have recoiled at this failure to appreciate the difference between passive and active roles; Catullus, writing of his boss, employs the useful Latin insult irrumator, which means "someone who forces others to give him oral sex," hence "one who treats people with contempt."