First of all, I want to mention that weasels are not rodents. In fact, they belong to a different taxonomic order than the rats and mice family. Their physical appearance is similar though: they have pointy snouts, small eyes and big ears. They have sharp teeth and their body is usually in a torpedo form with a long tail.

These creatures have been known since ancient times but there is still no set consensus when it comes to their classification. It seems that scientists can’t agree on whether weasels should be included among the minks or polecats (though technically speaking, minks are very closely related to weasels).
Weasel tattoos can mean many different things
Weasels are small and agile hunters, they live in burrows and prefer forest areas. Because of their appearance weasel tattoo can mean many different things: deceitfulness, agility and evilness (mostly because weasels were considered witches familiars).
The evil image is not that old though, as most people think. Weasels were once loved by the Europeans so much that families would even have them as their crests!
During the middle ages such practices went out of fashion and weasels became associated with all sorts of bad omens – probably due to the way they attacked chickens and other birds which was seen as a threat at that time. Since then weasel meaning turned into something completely opposite: dishonesty, treachery and over-permissiveness (the meaning of the idiom ‘letting someone off too easily’).
Lately, weasel tattoos have made their way into many tattoo studios. This doesn’t come as a surprise because they seem to be very popular among people who want tattoos that are unusual yet not extremely weird.