tight-lipped
adjective
• with the lips firmly closed, especially as a sign of suppressed emotion or determined reticence.
• "she stayed tight-lipped and shook her head"
Similar:
reticent,
taciturn,
uncommunicative,
unforthcoming,
unresponsive,
close-mouthed,
close-lipped,
silent,
quiet,
unspeaking,
guarded,
secretive,
private,
withdrawn,
of few words,
untalkative,
mum,
