The Collaborative International Dictionary of English v.0.48:

Guise \Guise\ (g[imac]z), n. [OE. guise, gise, way, manner, F.
   guise, fr. OHG. w[imac]sa, G. weise. See Wise, n.]
   1. Customary way of speaking or acting; custom; fashion;
      manner; behavior; mien; mode; practice; -- often used
      formerly in such phrases as: at his own guise; that is, in
      his own fashion, to suit himself. --Chaucer.
      [1913 Webster]

            The swain replied, "It never was our guise
            To slight the poor, or aught humane despise."
                                                  --Pope.
      [1913 Webster]

   2. External appearance in manner or dress; appropriate
      indication or expression; garb; shape.
      [1913 Webster]

            As then the guise was for each gentle swain.
                                                  --Spenser.
      [1913 Webster]

            A . . . specter, in a far more terrific guise than
            any which
            ever yet have overpowered the imagination. --Burke.
      [1913 Webster]

   3. Cover; cloak; as, under the guise of patriotism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

guise
    n 1: an artful or simulated semblance; "under the guise of
         friendship he betrayed them" [syn: guise, pretense,
         pretence, pretext]

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "guise":
   MO, action, actions, activity, acts, address, affectation, air,
   algorithm, alibi, angle, apology, apparel, appearance, approach,
   array, aspect, attack, attire, bearing, bedizenment, behavior,
   behavior pattern, behavioral norm, behavioral science, blanket,
   blind, brow, carriage, cast, cast of countenance, cloak, clothes,
   clothing, coat, color, coloring, complexion, comportment, conduct,
   configuration, costume, countenance, course, cover, cover story,
   cover-up, coverage, covering, covert, coverture, cowl, cowling,
   culture pattern, curtain, custom, demeanor, deportment, device,
   disguise, doing, doings, drape, drapery, dress, dressing, duds,
   effect, eidolon, excuse, facade, face, facet, facial appearance,
   false front, fashion, fatigues, favor, feathers, feature, features,
   feint, fig, figure, folkway, form, front, garb, garments, gear,
   gestalt, gestures, getup, gloss, goings-on, habiliment, habit,
   handle, hanging, hood, housing, image, imago, impression,
   investiture, investment, lame excuse, light, likeness, line,
   line of action, lineaments, linen, lines, locus standi, look,
   looks, maintien, manner, manner of working, manners, mantle, mask,
   means, method, methodology, methods, mien, mode, mode of operation,
   mode of procedure, modus operandi, modus vivendi, motions,
   movements, moves, observable behavior, order, ostensible motive,
   outfit, pall, pattern, phase, phasis, physiognomy, poise,
   poor excuse, port, pose, posture, practice, praxis, presence,
   pretense, pretension, pretext, procedure, proceeding, process,
   protestation, public motive, put-off, rags, raiment, reference,
   refuge, regard, respect, rig, robes, routine, screen, seeming,
   semblance, setout, sham, shape, shelter, shield, show, showing,
   shroud, side, simulacrum, slant, smoke screen, social science,
   sportswear, stalking-horse, stance, stratagem, style, subterfuge,
   system, tack, tactics, technique, tenor, the drill, the how,
   the way of, threads, togs, toilette, tone, total effect, traits,
   trick, trim, turn, turnout, twist, varnish, veil, vestment,
   vesture, view, viewpoint, visage, way, way of life, ways, wear,
   wearing apparel, wise