2 people who are having intimate relations but keeping it out of the eye of the public. Your secret lover is the person you are having relations with but not letting your friends find out about.
ex: Until discovered Monica and Bill Clinton were secret lovers.
When the party your friends throw is full of mostly male populates. Every one at the party has a "Jimmy". Or the only people invited were Jimmys (male). Or possibly more common, a guy named Jimmy invited all guys.
Your friend invites people to your house but only guys come. You say it looks like we are having a Jimmy Party