Watchword is a compound of watch (Old English wæcce 'a watching, guard') and word, originally a password given to a sentry on watch. In current use it is a noun meaning "a word or phrase serving as a guiding principle or as a password". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The surface form is compositional: its joined elements remain visible in the modern word. The historical chain includes Old English wæcce and Old English word. Related English forms include watch and word. The word's history is thus visible in its construction: the components remain interpretable even after the whole expression has settled into a conventional English meaning.