means The day immediately before today.

from From Old English 'geostran dæg' — literally 'yester day.' The 'yester-' part traces back to a Proto-Germanic root meaning 'the day before,' with possible distant cousins in Latin 'heri' and Greek 'chthes' (both also meaning 'yesterday'), pointing to a shared Proto-Indo-European ancestor. 'Yester-' once roamed more freely in English — you could speak of 'yestereve' or 'yestermorning' — but over centuries it shrank until 'yesterday' was nearly the only place it still lived.