Lump of brain

 is an item that the Lone Wanderer finds in the Point Lookout (add-on) for Fallout 3.

Location
The item is in the Engine Room of the Duchess Gambit, on the table next to Tobar. You can only get in this room after completing the quest Walking with Spirits and speaking to Nadine in the The Ark & Dove Cathedral. She will tell you that after you passed out in the Sacred Bog, someone cut your head open and removed a piece of your brain. Ask her further on the topic and she'll tell you to meet her at the Duchess Gambit in two days and she'll have something for you. When you meet her there, she'll reveal that it was actually Tobar who does the "surgery" in a deal with the tribals. She locked him in the Engine Room for you in case you should feel the need for revenge. Head in there and the two (Lone Wanderer and Tobar) will have a short conversation, and Tobar will automatically turn hostile. Dispatch him and loot him for quite a few items and caps, among of which is the unique clothes Grifter's Fit. Once he's dead, turn to your right and look at the small table in front of you and activate the jar to get a message and to put it in your inventory.