Whatchamacallit as a noun is a metasyntactic term used for any object whose actual name you don't know or can't remember. there are many spellings and many synonyms. while Thingummy as a noun is something one does not remember the name of
Part of speech: noun
Definition: a metasyntactic term used for any object whose actual name you don't know or can't remember. There are many spellings and many synonyms.
Part of speech: noun
Definition: something one does not remember the name of
We hope you now know whether to use Whatchamacallit or Thingummy in your sentence.