Synonyms for Whatchamacallit:

adj.

anonymous (adjective)
whatsis, X.

n.

so-and-so, whatsername, thingy, something, X. apparatus (noun)
stuff.
object (noun)
something.
whatchamacallit (noun)
sundries, whatsis, sundry, stuff.

Other synonyms:

thingy, whatsername. so-and-so. something. X. Other relevant words:
X, whatsername, thingy, so-and-so.