1 definition by Yogi Bhajan

Used to express astonishment when somebody says or does something completely out of character. The only possible explanation is that they've been replaced by an alien shape-shifter who looks exactly like them, but acts differently.

The phrase is a parody of the cliché found in old movies, when somebody returns home to find their lover gone and a stranger in their place.
Shanee: "I'm going to stay in tonight and study."
Ruthie: "Who are you, and what have you done with Shanee?"
by Yogi Bhajan February 9, 2011