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]

WordNet (r) 3.0 (2006):

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