What an unusual word jackanapes is! It is an archaic word which has been steadily declining in usage, though it has had a recent surge in popularity, possibly due to George R. R. Martin's use of the word in his very popular Game of Thrones.
But what does this word mean? Where did it come from?
The meaning is easy--much easier than the word origin question! A "jackanapes" is an impertinent or impudent person. Thus the Martin quote: "By his extreme youth, he can only be a prancing jackanapes, and so I name him."
If you want a mental picture to go with the word, picture a monkey on a leash used by an organ grinder. Full of mischief, and hard to control.
But where did this word come from? That's a great question--thank you for asking! There seems to be a variety of theories about where the word came from, as listed below. Before we begin, though, keep in mind that the name Jack is the diminutive for the very popular name John, and is widely used in fairy tales and nursery rhymes (Jack and the Beanstalk, Little Jack Horner, etc.). Jack became the equivalent of the "everyman." Think about the fact that the "Jack" in a deck of playing cards was once called the "knave."
Incidentally, William de la Pole, who is generally considered to be the "original" jackanapes, was banished and then beheaded, so if you were thinking of leading a life of jackanapery (yes, that really is a word!), you might want to reconsider...