thingumajig

Example:I put a thingumajig in the machine to start it.

Definition:A colloquial term for an object or part whose name has been forgotten or is unknown

Example:The thing-a-majig holds the sandwich together.

Definition:Another colloquial term for an object or part whose name is unknown or forgotten

Example:I know it's on the table somewhere, but I can't remember the whatchamacallit's name.

Definition:A term used for an object whose name is unknown or forgotten

Example:He picked up the doohickey and examined it closely.

Definition:A colloquial term for an object of unknown or unimportant name

Example:The gizmo fitted perfectly into the machine.

Definition:A device or tool whose name is unknown or unimportant