countersign

[ noun, verb koun-ter-sahyn; verb koun-ter-sahyn ]

/ noun, verb ˈkaʊn tərˌsaɪn; verb ˌkaʊn tərˈsaɪn /

a sign used in reply to another sign.

Military. a secret sign that must be given by authorized persons seeking admission through a guarded area.

a signature added to another signature, especially for authentication.

to sign (a document that has been signed by someone else), especially in confirmation or authentication.

Origin of countersign

1585–95; counter- + sign, modeled on Middle French contresigne, or its source, Old Italian contrasegno

The fellow was probably all right; his returning with the countersign would indicate it.

The Story of a Common Soldier of Army Life in the Civil War, 1861-1865 |Leander Stillwell
"Advance, one friend, and give the countersign," was the next command.

George at the Fort |Harry Castlemon
The house being between the lines the women connived in trying to get them captured; countersign is "Perth."

Personal Recollections and Civil War Diary, 1864 |Lemuel Abijah Abbott
So when she asked me the countersign, "Who's your favorite poet?"

Perkins of Portland |Ellis Parker Butler

To Merritt it sounded as if the words just spoken were a sort of countersign.

The Boy Scouts at the Panama Canal |John Henry Goldfrap

countersign

(tr) to sign (a document already signed by another)

Also called: countersignature the signature so written

a secret sign given in response to another sign

mainly military a password