[i. e. a Welch rare-bit] Bread and cheese toasted. See Rabbit.—The Welch are said to be so remarkably fond of cheese, that in cases of difficulty their midwives apply a piece of toasted cheese to the janua vita to attract and entice the young Taffy, who on smelling it makes most vigorous efforts to come forth.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Welch Mile * Welch EjectmentNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes