To say one thing while meaning another, often the opposite. The true meaning is left to whoever reads or hears.
In Hamlet
'The Mousetrap.' Marry, how? Tropically. [...] Your Majesty, and we that have free souls, it touches us not
Hamlet says that the play does not touch those who have “a free soul,” meaning the opposite: whoever feels touched gives himself away. The praise of Claudius’s innocence is the bait.
Act III: The Mousetrap and the death of Polonius, Hamlet, to Claudius