Middle English word tonight comes from Old English niht, Old English tō, and later Old English to niht (Tonight.)

Detailed word origin of tonight

Dictionary entry	Language	Definition
niht	Old English (ang)	Darkness.. Day, when computing spans of time.. Night, particularly:. The time between sunset and sundown.
tō	Old English (ang)
to niht	Old English (ang)	Tonight.
tonight	Middle English (enm)

Words with the same origin as tonight