tight–lipped 
/ˈtaɪtˈlɪpt/ adjective
/ˈtaɪtˈlɪpt/ adjective [more tight–lipped; most tight–lipped] 1 : not willing to speak about something
▪ He remained tight-lipped about his plans. ▪ tight-lipped witnesses 2 : having your lips pressed tightly together because you are thinking hard, angry, etc.
▪ He was tight-lipped in concentration.
▪ He remained tight-lipped about his plans. ▪ tight-lipped witnesses
▪ He was tight-lipped in concentration.
