Source : WordNet®

thingumajig
     n : something whose name is either forgotten or not known [syn:
         {dohickey}, {dojigger}, {doodad}, {doohickey}, {gimmick},
          {hickey}, {gizmo}, {gismo}, {gubbins}, {thingamabob}, {thingumabob},
          {thingmabob}, {thingamajig}, {thingmajig}, {thingummy}]