i don't think anyone knows what dahguy really means when they use it, but its really just another way of saying tugeye (dick), but it sounds more chill and less rez.
You call someone it when they being stoopid, or when you want them to bring you a Bepsi.
Theres also different ways of spelling it: duhgay, dug, tug, duh.

when you're hungover as shit after a spending a night drunk as a skunk