(v.) to yawn
To oscitate is to yawn. That’s one of those formal words for natural body processes coined in the more classically-informed seventeenth and eighteenth centuries, alongside the likes of eructate (to belch), pandiculate (to stretch with tiredness), and of course crepitate (to fart).
In the case of oscitate, the root here is the Latin word for the mouth, os, to which has been added the citare (meaning ‘to incite’ or ‘stir’), which itself comes from the Latin ciere (meaning ‘to move’). Despite appearances, that makes this word a none-too distant relative of the word oral, as well as usher (with os being used in a figurative sense, to mean an entranceway or doorway), and Aurgia (the constellation of a charioteer, connected here via the reins in the horses’ mouths).
The word oscitate itself dates back to 1623 in English, when it first appeared in the influential lexicographer Henry Cockeram’s English Dictionarie. It’s never been a particularly well-used word since, but if you’re yawning in polite company and wish to excuse your tiredness it might prove the mot juste.