watchword

Etymology

From Middle English wacche word, wacchworde, morphologically as watch (“guard, sentinel, sentry”) + word.

noun

1. A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry.
   For-ward! be our watchword, Steps and voices joined; / Seek the things before us, Not a look behind. 1871, “Forward! Be Our Watchword”, Henry Alford (lyrics), Henry Thomas Smart (music)
   Atrocities of the most vicious kind were justified by the watchwords: "This is war!" "Might is Right." "Necessity knows no law." 1919, Boris Sidis, The Source and Aim of Human Progress
   We can repay the debt which we owe to our God, to our dead, and to our children only by work — by ceaseless devotion to the responsibilities which lie ahead of us. If I could give you a single watchword for the coming months, that word is: work, work, and more work. 1945 May, Harry S. Truman, Announcement of Germany's Surrender
   It's a peel out the watchword / Just peel out the watchword 1994, Tori Amos (lyrics and music), “Cornflake Girl”, in Under the Pink
   The Esk Valley route to Whitby was a classic example: a basic four trains a day service has persisted for decades, with economy the watchword. 2019 October, James Abbott, “Esk Valley revival”, in Modern Railways, page 76
2. (military, security) A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies.
   a Watchword sufficient for him that is wiſe 1625, George Sandys, Sacrae heptades