Definition:

To sell very quickly; a characteristic of a popular product.

Example:

1)      My new book, "How to Make Millions in the Slang Business," is selling like hotcakes!

2)      The new Pokemon game sold like hotcakes.

Etymology:

A 'hotcake' is a pancake -- flour and egg and milk cooked in hot pan. It's not clear why 'hotcakes' are used as a model of popularity and high demand. It's probably because hotcakes taste best when they are fresh and hot, so when someone is cooking 'hotcakes' you have to run to get them as they're coming off the stove.