Josh Brolin Jokes About Nude Thanos, Rebukes 'Avengers: Endgame' Spoilers

Josh Brolin's dropping his evil villain persona and showing off his playful side to talk Thanos' ass cheeks, and a major no-no in his book -- "Avengers: Endgame" spoilers ... just don't do it! Brolin, who plays Thanos was at LAX Monday when we…