Knock-Knock is a Pulaski axe used by the Searchlight fire department. A unique and more powerful version of the fire axe, Knock-Knock's has a matte black handle and a polished steel head with a long adze on the back. Knock-Knock also swings faster than the fire axe, adding onto its impressive DPS.
Knock-Knock has a special move (Melee Weapons skill of 50) in V.A.T.S. called Mauler, which can cause knockdown, but costs significantly more Action points. Mauler will sometimes knock a weapon out of an enemies hand, or tear a limb off with a single hit instead of knocking them down.
It is found in Camp Searchlight's Fire Station. Once you get the key from Logan, head upstairs and search the bathroom stalls, look in the right side corner of the stall with the skeleton to find Knock-Knock.
Although you receive this weapon in rather poor condition, it has very high HP and will be able to be used for a long time before breaking. Additionally, it an be repaired with the axes in the garage downstairs.