Reading Catullus I’ve been wondering. He has various sexual swear words/insults, which seem to be quite specific. But how on earth do we know what word goes with what act? Has an ancient sex manual survived? Are there drawings with captions somewhere? Or has a scholiast provided a helpful gloss?
Of course people never stopped speaking latin, the language has never been extinct. So no ancient dictionaries had to be dug up to understand the manuscripts.
Also, many of the Latin profanities are borrowed from Greek which leads to a very similar question. Kasper is probably correct, though it is certainly not unheard of for a word’s meaning to be lost. It is a wonder we know the more specific meanings of Catullus’ naughty words, since old dictionaries like to lump them together under the vague category of “unnatural vices”. Even if a word’s meaning were to be lost, modern methods can sometimes provide clues based on word roots and comparisons to other words in the same language and other languages (I don’t know of any examples of a word being recovered in this way). paedicare for example, whether it derives from [size=150]παιδικος[/size] or Latin roots referring to shame (pudor, pudet, etc.), there is little doubt in the former what is going on when you “make someone your boy” or in the latter when you “take away someone’s dignity sexually”. I really don’t know for sure, and sorry for the graphic examples! 
that’s a very interesting question, I asked myself this one time.
I still don’t know how do we know the precise meanings, but Kasper has a valid answer (I think, I don’t know when did people started translating the ancient manuscripts - if it were when Latin was still spoken or not).
benissimus, what is paedet?
and that greek paidikos, is it “boyish, childish” only, without any sexual meaning?
and where did “irrumo” come from?
benissimus, what is paedet?
sorry, that was a cross between pudet and taedet. I could have sworn I edited that out before…
as far as I know, paidikos has so sexual meaning in Greek, but I don’t know much about that. maybe pedicare is really from pathicus, but that is just a guess. the meaning of irrumo is quite obvious from its formation: in + ruma (variant of rumen) “into the throat”, but verbalized.
Septuaginta, Genenis 26:8
“paraku/yas de\ Abimelec o( basileu\s Gerarwn dia\ th~s quri/dos ei)~den to\n Isaak pai/zonta meta\ Rebekka th~s gunaikos\s au)tou~.”
Paizonta is certainly held to have a sexual meaning here, as Abimelec protests that Isaak has told them Rebekka is his sister, and they have all been “paizonta-ing” with Rebekka.
A very good guess, in my humble opinion. 
whitaker’s WORDS lists irrumo, irrimo and inrumo - the latter propably is earlier. But it doesn’t list anything like “rumen” 
. I’ve found a few websites suggesting that “rumo” comes from something like “ruma” which would be “teat”, and the whole word would mean “offer the teat”. you can follow here (click) for the links I found.
The LSJ http://perseus.mpiwg-berlin.mpg.de/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2376679 gives play like a child as the basic meaning of παιζομαι with ‘play amorously’ as a subsidiary meaning. Your Genesis citation is one of the references they give. It seems to be a lot less specific than paedicare, but then it could be that the LSJ is just being coy. I don’t know.
The Perseus L&S has rumen http://perseus.mpiwg-berlin.mpg.de/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3D%2341990 and glosses it as throat, gullet.
this might be a silly digression, but how old is the latin word “condomum”?
“condomum” is not a Latin word.
Close this thread it’s disgusting
condomum, condomi N N [HBXDK] Modern lesser
condom;
source: Whitaker’s WORDS list
[HBXDK] =
H – modern – Coined recently, words for new things (19-20 [centuries])
(SOURCE=K) is especially noteworthy in that it is of modern, 20th century Latin and its meanings should probably not be applied to earlier texts.
source: Whitaker’s WORDS wordsdoc.htm 
My new dictionary does have the word “ruma, rumae” or “rumen, ruminis” (it really lists it like that - with the “or” and the “rumen” alternative) and then it says that it means both throat and teat.
I love this new dict, it marks all vowels’ duration. 
Thanks!