have something up one's sleeve

ALCOM WorldにPaperback Clubがあり、余り活発なクラブではありませんが、つい最近、Charlie and the Chocolate FactoryとKafka on the Shoreを読んだ感想が投稿されていました。後者は６００ページ以上もある長編なので敬遠して、前者の児童書を読むことにしました。　児童書でも結構知らない単語、表現が出てくることは経験がありますが、Charlie and the Chocolate Factory（以降はChocolate Factoryと書きます）には余り見掛けない単語は出てきませんが、知らない慣用句はちらちらと見掛けます。
It is, my dear, it is. And to all the kings and presidints of the world as well. But it isn't only candy bars that he makes. Oh, dear me, no! He has some really fantastic inventions up his sleeve, Mr. Willy Wonka has! Did you know that he's invented a way of making chocolate ice cream so that it stays cold for hours and hours without being in the icebox?
"has some really fantastic inventions up his sleeve"、この個所は、袖の中に隠して用意していると解釈しても、意味は通じますが、慣用句のようなので辞書で調べてみます。
・Random House Dictionary: to have a secret plan, scheme, opinion, or the like: I could tell by her sly look that she had something up her sleeve.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: Fig. to have a secret or surprise plan or solution (to a problem). (Alludes to cheating at cards by having a card hidden in one's sleeve.) I've got something up my sleeve, and it should solve all your problems. I'll tell you what it is after I'm elected. The manager has an ace up her sleeve. She'll surprise us with it later.
トランプでインチキをするところから出てきた表現ですね。