The Collaborative International Dictionary of English v.0.48:

Sidelong \Side"long`\, adv. [See Sideling, adv.]
   1. Laterally; obliquely; in the direction of the side.
      [1913 Webster]

   2. On the side; as, to lay a thing sidelong.

   Note: [See Sideling, adv. ] --Evelyn.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sidelong \Side"long`\, a.
   Lateral; oblique; not being directly in front; as, a sidelong
   glance.
   [1913 Webster]

         The bashful virgin's sidelong looks of love.
                                                  --Goldsmith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sidelong
    adv 1: on the side; "the plow lay sidelong on the ground"
    2: with the side toward someone or something; "seated sidelong
       to the window"- Nathaniel Hawthorne
    3: to, toward or at one side; "darting eyes looking sidelong out
       of a wizened face" [syn: sidelong, sideways, obliquely]
    adj 1: (used especially of glances) directed to one side with or
           as if with doubt or suspicion or envy; "her eyes with
           their misted askance look"- Elizabeth Bowen; "sidelong
           glances" [syn: askance, askant, asquint, squint,
           squint-eyed, squinty, sidelong]
    2: situated at or extending to the side; "the lateral branches
       of a tree"; "shot out sidelong boughs"- Tennyson [syn:
       lateral, sidelong]
    3: inclining or directed to one side; "moved downward in a
       sidelong way" - Bram Stoker

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "sidelong":
   aside, askance, askant, aslant, aslope, asquint, at an angle,
   atilt, away, backhand, backhanded, bevel, beveled, bias, biased,
   broadside, broadside on, canting, careening, circuitous,
   circuitously, covert, crabwise, deflectional, deviant, deviately,
   deviative, devious, deviously, digressive, digressively,
   divagational, divagationally, divergent, divergently, edgeway,
   edgeways, edgewise, excursive, excursively, flanking, glancing,
   glancingly, in juxtaposition, inclinational, inclinatory, inclined,
   inclining, indirect, indirectly, laterad, lateral, laterally,
   leaning, lee, leeward, left-handed, listing, nearby, next-beside,
   oblique, obliquely, on its side, on one side, on the beam,
   on the side, out of plumb, out of square, pitched, raking,
   recumbent, right and left, shelving, shelvy, side, sideling,
   sideward, sidewards, sideway, sideways, sidewise, sidling,
   sinister, sinistral, skirting, slant, slanted, slanting, slantways,
   slantwise, sloped, sloping, surreptitious, tilted, tilting, tipped,
   tipping, tipsy, to one side, to the side, weather, windward