Thingummy

Hat stretcher

Work, the what’s-its-name of the thingummy and the thing-um-a-bob of the what d’you-call-it.
– P. G. Wodehouse

Fortunately, I had all this wreckage to build a space swapping doodah-thingy-whatsit.
– The Sarah Jane Adventures (TV series)

English has several words for a thing or person whose name you don’t know or can’t remember or can’t be bothered to say. For example:

  • whatchamacallit
  • whatsit
  • gadget
  • doodah
  • thingy
  • thingamajig
  • thingummy
  • so-and-so
  • oojamaflip
  • what’s-her-name and what’s-his-name

There are other examples in the OED and Macmillan Thesaurus and Wiktionary. You can also read about vague expressions.