keep up one's sleeve

The Quiet Americanに5/15/2013に取り上げた慣用句（"have something up one's sleeve"）が出てきました。
'You'll never take me seriusly, will you, Thomas?' he complained, with that schoolboy gaiety which he seemed to have kept up his sleeve for this night of all nights, 'I tell you what--Phuong's at the cinema--what about you and me spending the whole evening together? I've nothing to do now.' It was as though someone from outside were directing him how to choose his words in order to rob me of any possible exuse. He went on. 'Why don't we go to the Chalet? I haven't been there since that night. The food is just as good as the Vieux Moulin, and there's music.'