1) An (often putative) upcoming event involving much sexual activity.
2) A sex party.
3) A party with the swapping of partners for sexual purposes.
4) Intercourse involving two or more people, usually suggesting that coupling has taken place several times.
1) I'm hoping for a shagfest on Friday. Marion has asked her friend over.
2) The vicar invites you to a shagfest after the main service on Sunday. Please bring your own mugs.
3) I picked up a sexy beast at the shagfest last night.
4) I didn't sleep last night: it was a complete shagfest and my dick is now sore with exertion.