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]

WordNet (r) 3.0 (2006):

underscore
    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]

The Free On-line Dictionary of Computing (19 January 2023):

underscore

    _, ASCII 95.

   Common names: ITU-T: underline; underscore; underbar; under.
   Rare: score; backarrow; skid; INTERCAL: flatworm.

   See also left arrow.

   (1995-03-06)