...here the two friends burst into a variety of giggles, and glanced from time to time, over the tops of their pocket-handkerchiefs, at Nicholas, who from a state of unmixed astonishment, gradually fell into one of irrepressible laughter.
An irrepressible person is lively and energetic and never seems to be depressed. Jon's exuberance was irrepressible. + irrepressibly ir·re·press·ibly Gavin was irrepressibly rebellious. full of energy, confidence, and happiness so that you never seem unhappy