The name given to an imaginary girlfriend/boyfriend, who is made up in order to avoid an awkward conversation between you and another person who wishes to ask you out, but whom you do not wish to go out with. The Foxy Toxy acts as a simple rejection tool, as the person who wished to ask you out then realises you already have a boyfriend or girlfriend, and doesn't ask you out - hence avoiding the awkward conversation. I mean, you could run away, too, but the Foxy Toxy is the best idea.
"Oh no! X is coming!"
"So?"
"He wants to ask me out, but I don't want to go out with him!"
"So?! Just make up a Foxy Toxy, and he'll get the message."
"She already has a boyfriend."
"Nah, she doesn't. She just has a Foxy Toxy."