— blank
— close
— closelipped
— closemouthed
— deadpan
— expressionless
— impassive
— inarticulate
— incommunicado
— inexpressive
— mum
— out of touch
— poker-faced
— secretive
— silent
— straight-faced
— taciturn
— tightlipped
— unarticulate
— unexpressive
— unpronounceable
— unutterable
— vacuous