'anight' originates from Old English, from the prepositional phrase 'on niht' ('on/in night'), where the unstressed prefix 'a-' developed from 'on' and combined with 'night'.

'on niht' in Old English became Middle English forms such as 'a nyght'/'anight', and eventually yielded the modern (now archaic/dialectal) English 'anight'.

Initially it meant simply 'at night', and this meaning has remained, though the form has become archaic or dialectal in modern usage.