To shew ostentatiously. To flash one’s ivory; to laugh and shew one’s teeth. Don’t flash your ivory, but shut your potatoe trap, and keep your guts warm; the Devil loves hot tripes.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Flash-1 * To Flash the HashNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes