Nugae! Ineptiae sunt aniles! Fabulae, logi, somnia! Gerras loqueris; hariolaris, vaticinaris!
- Nugae, ineptiae, gerrae are dedicated terms for nonsense, balderdash, trifles, idle speech, silliness, folly, … (I could go on). The expression ineptiae aniles means something like old wives' tales (also known as fabellae aniles).
- Fabulae should be clear – licet fabulas narrare, nisi si veritas exspectatur.
- Logi literally means words, but in the plural actually idle words, rubbish.
- Somnia literally means dreams, musings, but also delusions, woolgathering. I would not use it to accuse someone of intentional fabrication or at least careless untruthfulness, so it is probably not an appropriate translation of “bullshit.”
There are also some verbs:
- Hariolari literally means to prophesy, divine, which (or so it is my understanding) was actually a respectable occupation in ancient Rome, nevertheless it also means to speak foolishly, to talk silly stuff, nonsense.
- Vaticinari means essentially the same thing as hariolari.
Disclosure: Most of these are taken from Smith & Hall's entry for nonsense, with some cross-checking in Lewis & Short and my copy of Menge-Güthling. Neither Lewis & Short (1849) nor Menge & Güthling (revised edition from 1983, goes back to 1907) would dream of using a word like “bullshit,” but rather read like thesauruses of charming mild-mannered chiding in English and German, respectively. But I assume that the basic desire to declare someone else's utterings complete and unredeemable folderol has remained the same throughout the centuries.
A nice example is the following dialogue from Terence's Phormia, Act 3, Scene 2. (Phaedria is in love with a slave but cannot afford buying her. He is trying to persuade her owner, Dorio, to hold off on selling her to a sailor while he raises money; Dorio is having none of it.)
Ph. Non mihi credis? (You don't believe me?)
Do. Hariolare. [= hariolaris]
Ph. Sin fidem do. (What if I give you my word.)
Ph. Foeneratum istuc beneficium tibi pulchre dices. (You will later say this favour earned you a pretty penny.)
Ph. Crede mihi, gaudebis facto: verum hercle hoc est. (Believe me, you will be glad you did this, I swear it's true.)