A weasel is a sneaky and sly person. Your weasel of a friend has a habit of "forgetting" his wallet every time he goes out to dinner with you.
You can call someone who cheats and lies a weasel, or you can use the word literally, to refer to the small furry mammal called a weasel. The furry kind of weasel is known for its short legs, long neck, and musky smell. In fact, the word's Proto-Germanic origin wisand carries the meaning of a stinky animal. The verb weasel means to sneak or evade, like when you weasel out of doing the dishes.