whatchamacallit - noun

something unspecified whose name is either forgotten or not known

whatchamacallit - thesaurus

doodad   doohickey   doojigger   gimmick   gismo   gizmo   gubbins   thingamabob   thingamajig   thingmabob   thingmajig   thingumabob   thingumajig   thingummy   whatchamacallum   whatsis   widget