Flashing

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late 18th century: from the earlier synonym flash (of unknown origin) + -ing1.

wiktionary

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(weatherproofing) From original flashing material of copper or aluminum that reflects flashes of sunlight.

etymonline

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flashing (adj.)

1540s, of light; present-participle adjective from flash (v.).

flashing (n.)

1791, "act of creating an artificial flood," verbal noun from flash (v.); also compare flash (n.2)). Meaning "indecent exposure" is by 1968 (see flasher). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but the sense connection is unclear and it might be an unrelated word.