Wiktionary
n. 1 A thing (used in a vague way to refer to something whose name one cannot recall, or that one is embarrassed to say) 2 A specific person (used in a vague way to refer to someone whose name one cannot recall)
Usage examples of "whosit".
General Whosit to confiscate the thing and bundle me off to one of those cabbage farms or what have you.
Flayed, whosit, Marsyas, enormous boring picture, Rubens is it, or Titian?
He would have read the Dutch papers, which announced pompously that Divisional Commissaire Whosit was in charge of the Mystery Slaying.
Leaving three men to stare at our retreating backs, Mr Whosit and I made our wavering way up the street and around the corner to the familiar jungle-backed wall.
Five times he got to the point where the cops come to wake him up, once he almost had the name, and three times he tried to substitute other actors for whosits, but it didn't work.