Green’s Dictionary of Slang

1. one who lures players into a corrupt casino, by stressing how often the bank there has been broken.

1730		Derby Mercury 14 Jan. 3/2: ‘List of Officers which are established in the most notorious Gaming Houses [...] A Flasher, to swear how often the Bank has been stripped.
1797		Sporting Mag. Sept. X 312/1: [as cit. 1730].
1804		Sporting Mag. May XXIV 125/1: [as cit. 1730].

2. the penis.

1917

Harry Champion [perf.] ‘Aunt Tilly’ 🎵 Oh Aunt Tilly! Don’t forget to shout. / And let me know when you can see me flasher pokin’ out.

3. an exhibitionist.

1967		L. Bruce Essential Lenny Bruce 209: You know those guys in the park, the flashers?
1974		G.F. Newman You Flash Bastard 120: Sneed wasn’t interested in the flasher by the tit-book shop.
1981		H. Gould Fort Apache, The Bronx 201: Somebody had even collared Porfirio the flasher [...] Occasionally a passing cop would admonish him to ‘keep it in his pants’.
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings n.p.: FLASHER — A depraved male or female deviant, who indulges in surprising people by exposing their genitals in public.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 146: Everywhere you look down here you got flashers on the beach and jigglers on the street.
1992		D. Jarman diary 7 Aug. Smiling in Slow Motion (2000) 185: Derek said he had seen a masked flasher in Hyde Park being jerked off.
2004		S.B. Greer Blood Sisters 92: He pulled down his pants and started stroking his penis. Barbara screamed and ran [...] It wasn’t the first time they’d seen a flasher.
2010		Sun. Times News Review 19 Dec. 16/3: A flasher opens his coat, revealing all.
2021		Twitter 10 Nov. 🌐 [W]e never used to get guys [...] with their tickle tackle in hand, waving it over the doormat. Any that did were [...] called flashers.