Akin to the word knackered.
I'm shagged out after last night.
1) To be exhausted from a hard days work.
2) Unable to get wood due to excessive knobbing.
3) A gay phrase used by douchebags to try and make them believe they're having more sex than you.
Ben: "Ooooh yeah, after yesterdays exploits, I'm shagged out!"
Kevin: "What, those men tire you?"