Sentence:  "He took the this-is-unheard-of-but-not-really-serious-of-course attitude of a street fakir whose mechanical <WSD>dingus</WSD> flops during a demonstration." 
Meaning of the word marked with <WSD> tags:  A gadget, device, or object whose name is either unknown, forgotten, or omitted for the purpose of humor; a thingamajig.