>>1441785idiom (noun) - a group of words established by usage as having a meaning not deducible from those of the individual words
>Something like burning the barn down to kill a few mice.Deducible, therefore not an idiom.
>Burning the village in order to save itAn oxymoron, but not an idiom.
Cutting off your nose to spite your face? (I don't think its an idiom either.)