I. ˈən-dər-ˌskȯr transitive verb
Date: 1771
1. : to draw a line under : underline
2. : to make evident : emphasize , stress
arrived early to underscore the importance of the occasion
3. : to provide (action on film) with accompanying music
II. noun
Date: 1901
1. : a line drawn under a word or line especially for emphasis or to indicate intent to italicize
2. : music accompanying the action and dialogue of a film