Piss Horn 

A semi erect penis , usually shows itself in the morning , when you first wake up. Often makes it difficult to urinate as your aim is slightly off and youre half asleep.

Shit man , just pissed all over the floor coz of this fucking pisshorn!

Piss Horn by James November 1, 2004

Pisshorn 

When you wake up in the morning with an erect penis and have to use the toilet, you have pisshorn.

-"Mate, I've got pisshorn!"
-"Use the toilet, then!"

Pisshorn by Proper Socks September 4, 2013