close-mouthed
From Longman Dictionary of Contemporary Englishclose-mouthedclose-mouthed /ˌkləʊs ˈmaʊðd◂, -ˈmaʊθt◂ $ ˌkloʊs-/ (also closed mouthed /ˌkləʊzd- $ ˌkloʊzd-/ American English) adjective TALK TO somebodynot willing to say much because you are trying to keep a secret SYN tight-lippedExamples from the Corpusclose-mouthed• Emma suspected some of the other children had harassed her, but even to Emma, Ruth had been close-mouthed.