Among the more colorful characters in Roman literature and mythology, Egnatius the Spaniard takes the proverbial cake. Maybe Egnatius would have been better off brushing his teeth with cake, since it at least tastes better than urine. (Why Lesbia agreed to kiss Egnatius is anyone's guess; I wouldn't kiss a guy who brushed his teeth with urine.)
Real ancient Romans and Greeks added crushed oyster shells and bones to toothpaste and perfected it. In the Islamic world, there was a good toothpaste recipe, though the ingredients are lost to time (Wikipedia, "Toothpaste").
I wonder why Egnatius is described as brushing his teeth with urine. That is so gross. I know the Romans used to clean their wool with urine (Wikipedia, "Urine"), but brushing your teeth with it? Not even a Roman, who considered it perfectly fine to watch people be killed for sport, would do that. Maybe Catullus was mad at Egnatius for stealing Lesbia (apparently, Egnatius was either Lesbia's most surprising "moechor" or her favorite "moecher," since the other two hundred ninety-nine aren't mentioned by name) and this is an exaggeration; maybe Catullus is venting about how he lost Lesbia to, among others, a mere foreigner (note, in poem 39, Egnatius is described as "unurbane," meaning not Roman and therefore inferior. Also, were beards fashionable for Roman men in Catullus' day? I don't think they were).
So, should we take this literally? I am struggling to find references to Egnatius outside of Catullus' Carmina, so I'm not sure. However, I think we should all take this cum grano salis and remember: Romans were wonderful exaggerators.