laughing up his sleeve

小説Momoから取り上げる表現は今日も熟語です。
By dint of skilful haggling, Nino had persuaded Salvatore to part with a radio in exchange, laughing up his sleeve to think that Salvatore was getting the worst of the bargain.
"laughing up his sleeve" とはどんな笑い方でしょうか？
・Oxford English Dictionary: Be secretly or inwardly amused: he must have been laughing up at his sleeve all the time I was interviewing him
・Dictionary.com: be secretly amused or contemptuous; laugh inwardly: to laugh up one's sleeve at someone's affectations.
"laugh inwardly" ですか。内心ほくそ笑む感じですね。