what·cha·ma·call·it 
/ˈwɑ:tʃəməˌkɑ:lət, ˈwʌtʃəməˌkɑ:lət/ noun
plural what·cha·ma·call·its /ˈwɑ:tʃəməˌkɑ:lət, ˈwʌtʃəməˌkɑ:lət/ noun [count] US, informal : something whose name you have forgotten or do not know : thingamajig
▪ I can't find the whatchamacallit that holds the door open.
▪ I can't find the whatchamacallit that holds the door open.
