1A yellow sash or ribbon worn as a badge of honour; specifically (a) the orange-yellow sash on which the Prussian Order of the Black Eagle was worn; (b) the yellow ribbon worn by members of the Order of the Sword, an order of military merit founded by King Frederick I of Sweden in 1748.
Now somewhat rare.
2US A yellow ribbon (originally one tied to a tree) displayed as a symbol of support for a person in dangerous circumstances away from home, and in hope of his or her safe return; specifically (a) such a ribbon used to indicate support for members of the military; (b) such a ribbon used to indicate support for hostages (originally and chiefly with reference to the Iran hostage crisis (1979–81)).
Probably originating from the song ‘Tie a Yellow Ribbon Round the Ole Oak Tree’ (1973) by Irwin Levine and L. Russell Brown.
Mid 17th century; earliest use found in Jeremy Taylor (d. 1667), Church of Ireland bishop of Down and Connor and religious writer. Originally from yellow + riband; subsequently from yellow + ribbon.