means to open the mouth wide and inhale deeply in an involuntary reflex, usually triggered by tiredness, boredom, or simply seeing someone else do it

from From Old English ginian / gānian, 'to gape, open wide,' rooted in a Germanic family that also gave us 'gap' and 'gape' — and tracing back to a Proto-Indo-European base meaning 'to yawn or gape' (ghei-), the same ancient root that surfaces in Latin hiare, 'to gape,' and Greek chaos, the primordial 'yawning void.' So when you yawn, you're briefly reenacting the opening of the universe.