From Longman Dictionary of Contemporary Englishkeep your hand inkeep your hand inPRACTISE/PRACTICEto do something that you used to do a lot, so you do not forget how to do it You should at least work part-time, just to keep your hand in. → hand
Examples from the Corpus
keep your hand in• He moved slowly and cautiously, and kept his hands in clear view.• Nevertheless, he continued with his finger covered in blood, keeping his hand in his pocket.• He liked to bow and ruin the effect by keeping his hands in his pockets.• He kept his hands in his pockets as they beat him.• He kept his hands in his pockets so he would not smash Tony Angotti in the face.• Although she has retired now, she keeps her hand in by giving her grandchildren music lessons.• The one-time gifted student kept his hands in his pockets as he listened to the brief proceedings.• He still comes around the gym occasionally, just to keep his hand in.• For all his pride, he misses the public forum politics provides and wants to keep his hand in somehow. 