The One Big Happy in today’s comics feed:
“Two negatives make a positive” is one way to state a principle of logic, that the negation of the negation of X is equivalent to X. The principle is irrelevant to an account of the syntactic phenomenon that’s popularly called “double negation” (or more generally, “multiple negation”) — often labeled negative concord by linguists — according to which all susceptible elements in a negated clause themselves appear in a negative variant (I didn’t see nobody nowhere, corresponding to standard English I didn’t see anybody anywhere); in languages or varieties or styles with negative concord, two negative elements are just the expression of one negation.
But Joe takes us into new territory, with his novel interpretation — actually, willful misinterpretation — of the principle of logic (or of algebra, as her father puts it): according to Joe’s interpretation, saying two negative (that is, deprecatory or insulting) things counts conversationally as saying something positive (that is, favorable or complimentary). All to take the opportunity to double down on nastiness.