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From The Collaborative International Dictionary of English v.0.48:
Underline \Un`der*line"\, v. t. 1. To mark a line below, as words; to underscore. [1913 Webster] 2. To influence secretly. [Obs.] --Sir H. Wotton. [1913 Webster] 3. Emphasize or call attention to; highlight; as, Long waits at the emergency room underline the need for a larger hospital. . [JG]