This question already has an answer here:
Is there a name for words like whatchamacallit, doohickey, thingamabob, doodad, thingamajig, whatsit, etc.?
Somehow it seems like there should be a word to describe the general term for a word that you use for referring to something when you cannot remember what it is called.
(Computing names like
baz are examples of metasyntactic variables but those are a slightly different category.)