Nightly

wiktionary

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From Middle English nyȝtly, nihtlich, nihtlic, from Old English nihtlīċ, nihtelīċ(“nocturnal, nightly, of the night, at night”), equivalent to night +‎ -ly. Cognate with Scots nichtlie(“nightly”), West Frisian nachtlik(“nightly, nocturnal”), Dutch nachtelijk(“nightly, nocturnal”), German nächtlich(“nocturnal, nightly”), Danish natlig(“nightly”), Swedish nattlig(“nightly, nocturnal”).

From Middle English nyghtly, neghtly, from Old English *nihtlīċe(“nightly”), equivalent to night +‎ -ly.

etymonline

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nightly (adj.)

Middle English nightli, from Old English nihtlic "nocturnal, at night, occurring during or characteristic of the night;" see night + -ly (1). As an adverb, "every night," Middle English nihtlich (mid-15c.), from the adjective.