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) inside 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 path till you reach the metal staircase. Go down two flights, go through the door on the east side, and follow the tunnel down and to the north. (There are two doors near the staircase leading to similar-looking tunnels. The incorrect tunnel will lead you to a room filled with Mirelurk egg clutches. The correct tunnel will be booby-trapped with frag mines). The room containing The Shocker is trapped with a rigged shotgun and tripwire in the doorway.