E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Monkey = the Devil;
an imp of mischief. Hence, a meddlesome child is spoken to as “you little monkey;” and is called “a regular imp,” or “imp of mischief.” The allusion is to the old drawings of devils, with long tails and monkey ugliness.
1
To get (or have) one’s monkey up. To be riled. Here the allusion is also to the devil or evil spirit in man; he will be “in a devil of a temper.” Even taken literally, monkeys are extremely irritable and easily provoked.
