Rascal, imp (2-3-2)
(Other definitions for so-and-so that I've seen before include "Difficult person - nobody in particular" , "What's-his-name; a nuisance" , "This or that person or thing" , "Disfavoured person" , "Unpleasant person - whose name has been forgotten!" .)
(Another definition for soandso that I've seen is " Unpleasant person - whose name has been forgotten!".)