thingumajig

/ˈθɪŋəməˌdʒɪɡ/
noun
  1. A word used when you cannot remember or do not know the name of an object.
    • She used a little thingumajig to open the watch.
    • I can't find the thingumajig that connects the hose.
    • What's that thingumajig on your desk called?