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]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "underline":
   accent, accentuate, band, bar, belabor, blaze, blaze a trail,
   blemish, blotch, brand, chalk, chalk up, check, check off,
   cicatrize, cross-hatching, dapple, dash, define, delimit,
   delineation, demarcate, diagonal, discolor, dot, dotted line,
   dwell on, emphasize, engrave, feature, fleck, freckle, gash,
   give emphasis to, hachure, hairline, harp on, hatch, hatching,
   highlight, impress, imprint, italicize, legend, line, lineation,
   make a mark, mark, mark off, mark out, mottle, nick, notch,
   overaccentuate, overemphasize, overstress, pencil, pepper,
   place emphasis on, play up, point, point up, prick, print, punch,
   punctuate, puncture, riddle, rub in, scar, scarify, score, scotch,
   scratch, seal, seam, slash, speck, speckle, splotch, spot,
   spotlight, stain, stamp, star, stigmatize, streak, streaking,
   stress, striate, striation, strip, stripe, striping, stroke,
   sublineation, tattoo, tick, tick off, trace, underlining,
   underscore, underscoring, virgule