показать кузькину мать

Etymology

 * lit. 'to show Kuzka's mother' (to someone). The proverbial is no-one in particular. It may be a pejorative short name for  or a dog's name. Possibly implying that little puppy Kuzka's mother is a big and vicious dog.
 * The possessive form is normally in lower case because it is a set expression.

Verb

 * 1)  to teach someone a lesson