Brewer's: Sanguinary James
(A). A sheep's head not singed. A jemmy is a sheep's head; so called from James I., who introduced into England the national Scotch dish of “singed sheep's head and trotters.” No real Scotch dinner is complete without a haggis, a sheep's head and trotters, and a hotch-potch (in summer), or cocky leekie (in winter).
A cocky leekie is a fowl boiled or stewed with leeks or kale—i.e. salt beef and curly greens. Gimmer (a sheep) cannot be the origin of Jemmy, as the G is always soft.