The Collaborative International Dictionary of English v.0.48:

Closemouthed \Close"mouthed`\, a.
   Cautious in speaking; secret; wary; uncommunicative.
   [1913 Webster]

WordNet (r) 3.0 (2006):

closemouthed
    adj 1: inclined to secrecy or reticence about divulging
           information; "although they knew her whereabouts her
           friends kept close about it" [syn: close,
           closelipped, closemouthed, secretive,
           tightlipped]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "closemouthed":
   brief, brusque, close, close-lipped, close-tongued, concise, curt,
   dark, discreet, dumb, economical of words, evasive,
   indisposed to talk, laconic, mum, mute, quiet, reserved, reticent,
   secret, secretive, shifty, short, silent, snug, sparing of words,
   speechless, taciturn, terse, tight-lipped, tongue-tied,
   uncommunicative, unloquacious, untalkative, word-bound, wordless