The Shocker is a sort of civilian device, as noted by the shocker glove holotape, but what it was meant to do or be is never explained. It is especially effective in combat against robotic enemies, dealing nearly double bonus damage, and causing a visual electrical effect similar to that left by pulse grenades or pulse mines, or other weapons that deal electrical damage.
The Shocker is found (complete with holotaped instructions) in the Flooded Metro west of Arlington Library; however, it is located much closer to the Mason District South entrance than the Flooded Metro entrance, and entering there avoids the majority of the metro's mirelurk population. From Mason District South, follow the obvious path to the open-area staircase. Go down two flights, turn due east, open the door, and follow the tunnel down and to the north. The tunnel will be booby-trapped with frag mines, and the room containing The Shocker is further trapped with a rigged shotgun and tripwire in the doorway.