The Collaborative International Dictionary of English v.0.48:

Slightly \Slight"ly\, adv.
   1. In a slight manner.
      [1913 Webster]

   2. Slightingly; negligently. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slightly
    adv 1: to a small degree or extent; "his arguments were somewhat
           self-contradictory"; "the children argued because one
           slice of cake was slightly larger than the other" [syn:
           slightly, somewhat, more or less]
    2: in a slim or slender manner; "a slenderly built woman";
       "slightly built" [syn: slenderly, slimly, slightly]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "slightly":
   a bit, a little, barely, by a hair, by an ace, ever so little,
   exiguously, extremely, faintly, fairly, feebly, frugally, hardly,
   imperfectly, in a measure, in a nutshell, in a way, in miniature,
   in some measure, in the small, inappreciably, inconsequentially,
   insignificantly, just a bit, kind of, lightly, little, marginally,
   meagerly, minimally, minutely, negligibly, not hardly, only just,
   poorly, pretty, punily, quite, rather, scantily, scarcely, small,
   smally, somewhat, sort of, sparely, sparingly, tant soit peu,
   to a degree, to some extent, triflingly, very, weakly