flashing (adj.) Look up flashing at Dictionary.com
1570s, of light; present participle adjective from flash (v.).
flashing (n.) Look up flashing at Dictionary.com
"indecent exposure," 1896, verbal noun from flash (v.). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but it is of unknown origin and might be an unrelated word.