Old English *beorhtnian "to make bright" (see bright (adj.) + -en (1)). Intransitive sense, "to become brighter," attested from c. 1300. Figurative use "dispel gloom from, cheer" is from 1590s. Related: Brightened; brightening. The simple verb bright (Old English byrhtan "be bright," geberhtan "make bright") was in Middle English, often in figurative senses "cleanse, purify; clarify, explain," but has become obsolete.
