The term 'watchword' is a compound formed from two native English elements, 'watch' and 'word'. Its earliest military sense, dating to the 16th century, was a literal 'word' given to sentinels on 'watch' to distinguish friend from foe, effectively a password. This concrete usage evolved logically into a more abstract meaning: a word or slogan that members of a group 'watch' or pay attention to as a guiding principle. Thus, the journey from a practical security command to a symbolic rallying cry perfectly encapsulates the compound's inherent logic of a 'word to be vigilant about'.

Structure: watch + word - watch: From Old English wæccan (to watch, be awake), of Germanic origin. Functional role: The core concept of vigilance or attention. - word: From Old English word, of Germanic origin. Functional role: The linguistic unit, the thing that is spoken or communicated.