The act of Receiving Fellatio while in a swimming pool, when your body is submerged, but your erect penis is sticking out of the water. Giving the appearance that your dick is a periscope.
Person 1: This girl was periscoping me in the pool last night mane.
Person 2: Nigga wats that?
Person 1: You know she was giving me head in the pool, I was underwater but when I put my dick above water that shit looked like a periscope.