Thing-um-a-jig!

He would answer to “Hi!” or to any loud cry,
  Such as “Fry me!” or “Fritter my wig!”
To “What-you-may-call-um!” or “What-was-his-name!”
  But especially “Thing-um-a-jig!”

See also: en.wiktionary.org/wiki/thingamajig and www.etymonline.com/search?q=thingamajig.

2020-02-25, update: 2022-04-19