Usually due to moderate or unsuccessful arousal or drunkeness.
Sometimes called a "Semi"
"After many beers Douglas was unable to get beyond Half Mast, Gemme was going to get a dud root tonight...if any!"
Shamus: Well I've been at Half Mast for about an hour now, so it should be here anytime now
Cordon: Have you been stuffing your pants?
Bleu: Na I've just been sailing at half mast for about a few minutes now
Dave (to himself): Wow who knew that flying half mast in public would be so easy to get away with.
Lady (casually looking at dave): Wow that guy is at half mast, someone should really tell him to chill out