A jackanapes is a smart-mouthed, rascally person. You might be tempted to call your babysitting charge a jackanapes after he locks you out of the house for the third time.
Jackanapes is a colorful but very old fashioned way to describe a rascal or a whippersnapper. Your great-grandfather might shake his cane and yell, "Get off my lawn, you jackanapes!" when the neighbor kids lose their basketball in his yard, but they're unlikely to know what the word means. Jackanapes is from the fifteenth century, and it's thought to come from the phrase "Jack of Naples," or to have some connection to the word apes, but experts are uncertain.