thingumabob
     n : something whose name is either forgotten or not known [syn:
         {dohickey}, {dojigger}, {doodad}, {doohickey}, {gimmick},
          {hickey}, {gizmo}, {gismo}, {gubbins}, {thingamabob}, {thingmabob},
          {thingamajig}, {thingumajig}, {thingmajig}, {thingummy}]
Source: WordNet® 2.0