The act of hiding your boner between your underwear and your belly so that it faces up 180 degrees, maintaining stealth. The head of the penis sticks out of the pants but is masked by the underwear, jeans, shorts, and/or shirt emulating a periscope. Using the periscope allows you to do everyday activities, while walking around with a massive errection. (for best results use with belt)
I felt nervous to solve the math equation in front of the class, but then I just used the periscope.
I hope parents don't get the wrong idea when I am at the daycare with my raging hard-on, thank God my step-dad taught me the periscope.