Tonight

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Old English tō niht, from the preposition to + night. Compare with today and tomorrow.

wiktionary

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From Middle English tonyght, to niȝt, from Old English tō niht.

etymonline

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tonight (adv.)

Old English toniht "in the coming night," from to "at, on" (see to) + niht (see night). As a noun, "in the night after the present day," early 14c. Written as two words until 18c., after which it was to-night until early 20c.