What is the meaning of the idiom a dog in the manger?

A 'dog in a manger' is a term often used towards someone who spoils enjoyment for others but gets no advantage themselves. Imagine a dog sitting in a manger preventing the horse from eating the hay. The dog cannot eat the hay itself but is resentlful of the horse enjoying its meal so it lays in the manger to spite the horse. These 'dogs' are usually jealous bitter people.