'anights' originates from Middle English, specifically the phrase 'a nyghtes,' where 'a-' meant 'on, in' and the suffix '-es' functioned as an adverbial genitive meaning 'of, during.'

'a nyghtes' in Middle English (from Old English 'on niht' and the genitive 'nihtes') evolved in Early Modern English to 'anights' and became the modern English adverb 'anights'.

Initially, it meant 'by night; during the night,' and this meaning has remained essentially the same, though the form is now archaic or dialectal.