The story
An allusion to the first line in Virgil's First Eclogue: "Tityre tu patulae recubans sub tegmine fagi" ("Tityrus, thou reclining beneath the shelter of the spreading beech tree").
Etymology from Wiktionary, CC BY-SA 4.0.
Tityre-tu
Today it means: A member of a gang of criminal youths that originated in the early 1620s. Full definition →