tight-lipped

• tightlipped

From tight +‎ lipped.

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tight-lipped (comparative more tight-lipped, superlative most tight-lipped)

1. (literal) Having the lips pressed tightly together, hence, not speaking.
   • 1922, Rafael Sabatini, Captain Blood: His Odyssy, chapter XXV:

     He would have sprung upon Captain Blood, who stood aloof, alert, tight-lipped, and watchful.

   • 1936, F.J. Thwaites, chapter XV, in The Redemption, Sydney: H. John Edwards, published 1940, page 162:

     Gaining the dak, they were joined on the veranda by four tight-lipped men.

2. (idiomatic) Unwilling to divulge information.
   • 2022 May 4, “Network News: RAIB looking into emergency brake application of train at Peterborough”, in RAIL, number 956, page 12:

     The open access operator has remained tight-lipped over what may have caused the driver to suddenly apply the brake, but stressed that safety was its priority.

• (unwilling to divulge information): closemouthed, discreet, secretive

• (antonym(s) of “unwilling to divulge information”): indiscreet, loose-lipped

• tight lips