countersign（n.）
"一种军事口令，是给予站岗士兵的信号，命令他不允许任何人通过，除非先给出那个信号"，1590年代，来自法语 contresigne，源自 contre- "反对"（见 contra-) + signe "标志"（见 sign（n.））。
COUNTERSIGN. A watchword used by military bodies as a precaution against an enemy or enemies. The countersign may be changed at any moment, or any number of times, but is usually altered each twenty-four hours. It is given primarily to commanders of guards, and outposts and their sentries, to reconnoitring and visiting patrols, and to the field and regimental officer of the day. All others desiring to pass through the lines must first be supplied with the countersign, which is thus a guard against spies, strangers, and surprise. ["New International Encyclopaedia," 1906]
口令。军队使用的警戒口令，以防敌人。口令可以随时更改，或更改任意次数，但通常每24小时更改一次。它首先给予警卫和哨兵的指挥官，侦察和访问巡逻队，以及现场和团队的当值军官。所有其他希望通过防线的人必须首先获得口令，这样就可以防止间谍，陌生人和突袭。["新国际百科全书"，1906]
该词起源时间：1590年代