The practice of urinating outdoors at a crowded house party so as to avoid the long line for the single indoor bathroom. The half bath can be situated in a garden, yard, driveway or other semi-secluded spot outdoors. Term originates from the name given to the second or guest bathrooms many newer houses are equipped with, but are absent in older houses and apartments.
When a person, guy or girl, goes to the bathroom to take a shit right after a guy has just pissed, leaving the toilet seat up, and the second person forgets to put the seat down and their ass hits the water.
Kasey - Dammit, Blake! You forgot to put the toilet seatback down! Now my ass is all wet!
Blake - Oh, I also forgot to flush....
Blake was then murdered for Half-Bathtize ing Kasey.