E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The head of a slaughtered sheep. There are boiled jemmies, baked jemmies, and sanguinary jemmies (raw sheeps heads). The tradition is that James IV. of Scotland breakfasted on a sheeps head just before the battle of Flodden Field (Sep. 9, 1513).
Mr. Sikes made many pleasant witticisms on jemmies, a cant name for sheeps heads, and also for an ingenious implement much used in his profession.Dickens: Oliver Twist.