What's his name?

Sarah (looking out the window): Oh, look! There's what's-his-name!
Me (getting up to look, seeing a stranger in the neighbor's yard): We don't know that guy.
Sarah: I know. That's why I called him "what's-his-name".
Me: "What's-his-name" refers to someone you know but whose name you can't remember. Not a complete stranger.
Sarah: Don't hold me to such a high standard.