zip-lippedhttp://www.oed.com/dictionary/zip-lipped_adj%3Ftab%3Dmeaning_and_usezip-lipped, adj.
First published 2021

zip-lippedadjective

  1. 1943–
    Having the lips tightly closed, esp. as a sign of suppressed emotion or determined reticence; (chiefly figurative) uncommunicative, determinedly reticent; cf. tight-lipped adj.
    1. 1943
      War Department Zip-Lipped About Army-Navy Game... The War Department had 'no comment' today regarding published reports.
      Greenville (Ohio) Daily Advocate 23 July 5/3
    2. 1982
      Stony-faced owners, zip-lipped owners..tend to have dull, disobedient dogs.
      B. Woodhouse, No Bad Dogs (1992) xii. 63
    3. 2019
      Information about his two grandchildren is rationed... These are the zip-lipped instincts of a former KGB officer.
      Times (Nexis) 28 September (Sat. Review) 16

Originally published as part of the entry for zip, int. & n.¹

zip-lipped, adj. was first published in 2021.