Source : Webster's Revised Unabridged Dictionary (1913)

Underline \Un`der*line"\, v. t.
   1. To mark a line below, as words; to underscore.

   2. To influence secretly. [Obs.] --Sir H. Wotton.

Source : WordNet®

underline
     n : a line drawn underneath (especially under written matter)
         [syn: {underscore}]
     v 1: give extra weight to (a communication); "Her gesture
          emphasized her words" [syn: {underscore}, {emphasize}, {emphasise}]
     2: draw a line or lines underneath to call attention to [syn: {underscore}]