thingamado

What you call something when you can't remember what it's called. See whatchamacallit

Person 1: "Hey where's that thingamado?"
Person 2: "The what?"
Person 1: "You know, that thingamado I was using earlier."
Person 2: "Ohhhhh, I know what you're talking about. Yeah the thingamado is next to the whatchamacallit."