'flashing' originates from Middle English, specifically the word 'flaschen' (also recorded as 'flashen'), where the root was imitative in origin, evoking a sudden burst of light or noise.

'flashing' changed from the Middle English verb 'flaschen'/'flashen' into Early Modern English as 'flash'; the modern present-participle form 'flashing' developed from that base, and the noun sense for metal strips (roof flashing) arose later by extension of the word for a brief or striking appearance.

Initially, it meant 'a sudden burst of light or noise'; over time it broadened to include 'to emit a brief light', 'to show something briefly', an adjective sense 'showy or conspicuous', and a noun sense 'metal strip for waterproofing'.