Laugh in One's Sleeve
(To). The French is: “Rire sous cape, ” or “Rire sous son bonnet.” The German is: “Ins faüstchen lachen”. The Latin is: “In stomacho ridere. ” These expressions indicate secret derision: laughing at one, not with one. But such phrases as “In sinu gaudere ” mean to feel secret joy, to rejoice in one's heart of hearts.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894