Os impurum, "filthy mouth" or "impure mouth", was a term of abuse especially for those who provided oral sex. "Oral turpitude" was a favorite form of invective for Catullus, Horace, and Martial. An accusation of having an os impurum is an "extreme obscenity," so vile that Cicero reserved it for men of lower standing than himself, only implying that their debasement tainted their more powerful patrons who were his real targets.
It was a convention of obscenely comic verse that oral sex caused bad breath that was nearly toxic. "Whores of the alleyways" are contaminated from giving oral sex; Catullus refers to "the foul saliva of a pissed-over whore." The urinary function of the penis makes oral sex particularly repulsive to Catullus, who elsewhere reviles a Celtiberian for brushing his teeth in urine. Martial jokes that a fine perfume turned to garum, fish sauce, when it was sniffed by a man whose breath was putrid from oral sex. In another of Martial's epigrams, a fellator breathes on a hot cake to cool it down and turns it to excrement. The bad breath and rotten teeth that are attributed to performing oral sex represent moral decay and a general corruption of the mouth's positive functions as the organ of a citizen's persuasive speech.
Read more about this topic: Sexuality In Ancient Rome, Sex Acts and Positions