thingamajig \ THING-uh-muh-jig \ ,
noun;
1. Informal. a gadget or other thing for which the speaker does not know or has forgotten the name.
Quotes:
“Why, a thingamajig . For dogs, you know.” Sally nodded. “Oh, a thingamajig for dogs? Now I understand.”
-- P. G. Wodehouse, "The Adventures of Sally," 1921
“You have to punch out the questions on that thingamajig , and the answers come out on tape from the whatchamacallits. You can't just talk to it.” A doubt crossed his fine face.
-- Kurt Vonnegut, "Player Piano," 1952
Origin:
The origins of thingamajig are unknown, though it first appeared in English in the 1870s. It is an extended form of the word thing that likely came from an earlier form, thingum .