Whatchamadingus (noun, humorous/slang):
A playful
mash-up of "whatchamacallit" and "
dingus", used to refer to a person, object, or gadget whose name you can't recall or
don't care to specify. Often used when pointing to something or someone with vague familiarity or comic effect.
Tone: Informal, whimsical, slightly old-fashioned or cartoonish.