What is Standby Stan?
1.
A man who one is able to fall back on as a last resort; someone who is kept in readiness to serve as a substitute.
Nancy: Hey, why don't you ever go out with So-and-so?
Claire: I don't really like him, I just keep leading him on in case things don't work out with What's-his-name.
Nancy: Oh, so he's your Standby Stan.
See