The Collaborative International Dictionary of English v.0.48:

Underscore \Un`der*score"\, v. t.
   To draw a mark or line under; to underline. --J. Tucker.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "underscore":
   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, 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, underline,
   underlining, underscoring, virgule