What does the expression “to get one’s monkey up” mean and Where does it come from?

The expression “to get one’s monkey up” means: To get one’s dander up; to become angry.

Apparently, along about a hundred years ago, people first began to compare their sudden fits of anger with the unreasoning bursts of rage so often seen among members of the simian family.

The expression is of British origin and, though sometimes heard on the western side of the Atlantic, is generally confined to the place of its origin.