tonight (adv.)youdaoicibaDictYouDict[tonight 词源字典]
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.[tonight etymology, tonight origin, 英语词源]