In vino Veritas. In Aqua satietas. In... What is the Latin for Tea? What! Is there no Latin word for Tea? Upon my soul, if I had known that I would have let the vulgar stuff alone.

Belloc’s joke turns on mock-classical pretension. He begins with the well-known tag “In vino veritas” (“in wine, truth”) and then tries to extend the formula to other drinks, only to be thwarted by tea’s lack of a satisfying “classical” Latin pedigree. The humor comes from treating Latin as a marker of cultural legitimacy: wine (and even water) can be dignified with Latin, but tea—associated in Edwardian/English life with everyday domesticity—cannot, so it is dismissed as “vulgar.” The line also satirizes pedantry and snobbery: the speaker’s allegiance is not to taste or health but to whether a thing can be wrapped in learned language.