Antic disposition means a frivolous or deliberate playfulness. It may also mean bizarre, irrational or threatening behavior closer to madness than madcap, as it does in Shakespeare's play, "Hamlet."
Hamlet disguises his quirky behavior as a clever charade, rather ridiculous actions that are out of place. He uses antic disposition to confuse the king and his allies as to his true intentions. Hamlet assures his mother that his madness helps him attain his objectives. "I essentially am not in madness, but mad in craft." He intends to exact revenge upon Claudius, who murdered his father, but he hides his true motives to avoid suspicion.