thingamajig

… dozens of multi-colored plastic thingies, metal gizmos in various shapes and a rubber whatchamacallit with a hole in the middle.

— Dick Wolfsie Joni had reached the point in her life when the wrong words flew out of her mouth. And when she tried to find the correct word, it inexplicably vanished. "Where's the whatchamacallit ?" — PRWeb.com

something that is hard to classify or whose name is unknown or forgotten