'brighten' originates from Old English, specifically the root 'beorht'/'beorh' meaning 'bright' combined with the verb-forming suffix '-en' (from Old English '-ian'), meaning 'to make or become'.

'brighten' developed in Middle English by adding the verb-forming suffix '-en' to the adjective 'bright' (from Old English 'beorht'); the sequence went roughly Old English 'beorht' -> Middle English 'bright' -> Middle English/early Modern English 'brighten'.

Initially, it meant 'to make bright' (in the literal sense of adding light or color), but over time it also acquired the figurative meaning 'to make or become more cheerful or hopeful.'