Latin: if there is no slang terminology utilized in it, how boring a language is Latin?

Quite apart from the sexual vocabulary noted by other respondents, Vulgar Latin, as we can reconstruct it from the Romance languages, had words we can only classify as slang.

Such as testa “head”, which originally meant “pot”. Or caballus “nag” instead of equus “horse”. Or using manducare “to chew” instead of edere for “to eat”. Or, instead of loqui “to speak”, using fabulare “to tell stories” or parabolare “to tell parables”.

For a lot more instances of reconstructed Vulgar Latin words, see Vulgar Latin vocabulary.