concrete cast on

A "concrete cast on" is another term used to describe a hard penis, BUT can only be used whilst traveling in a motor vehicle (or public transport of any sort).
It is most commonly used after someone has woken up from a short but tasteful nap, where an erection was not expected, or planned.

GUY 1: *asleep*
GUY 2: wake up dude, we've arrived at our destination
GUY 1: *wakes up* oh man I've got a raging concrete cast on right now!!