The Collaborative International Dictionary of English v.0.48:

Solemn \Sol"emn\, a. [OE. solempne, OF. solempne, L. solemnis,
   solennis, sollemnis, sollennis; sollus all, entire + annus a
   year; properly, that takes place every year; -- used
   especially of religious solemnities. Cf. Silly, Annual.]
   1. Marked with religious rites and pomps; enjoined by, or
      connected with, religion; sacred.
      [1913 Webster]

            His holy rites and solemn feasts profaned. --Milton.
      [1913 Webster]

            The worship of this image was advanced, and a solemn
            supplication observed everry year.    --Bp.
                                                  Stillingfleet.
      [1913 Webster]

   2. Pertaining to a festival; festive; festal. [Obs.] "On this
      solemn day." --Chaucer.
      [1913 Webster]

   3. Stately; ceremonious; grand. [Archaic]
      [1913 Webster]

            His feast so solemn and so rich.      --Chaucer.
      [1913 Webster]

            To-night we hold a splemn supper.     --Shak.
      [1913 Webster]

   4. Fitted to awaken or express serious reflections; marked by
      seriousness; serious; grave; devout; as, a solemn promise;
      solemn earnestness.
      [1913 Webster]

            Nor wanting power to mitigate and swage
            With solemn touches troubled thoughts. --Milton.
      [1913 Webster]

            There reigned a solemn silence over all. --Spenser.
      [1913 Webster]

   5. Real; earnest; downright. [Obs. & R.]
      [1913 Webster]

            Frederick, the emperor, . . . has spared no expense
            in strengthening this city; since which time we find
            no solemn taking it by the Turks.     --Fuller.
      [1913 Webster]

   6. Affectedly grave or serious; as, to put on a solemn face.
      "A solemn coxcomb." --Swift.
      [1913 Webster]

   7. (Law) Made in form; ceremonious; as, solemn war;
      conforming with all legal requirements; as, probate in
      solemn form. --Burrill. --Jarman. --Greenleaf.
      [1913 Webster]

   Solemn League and Covenant. See Covenant, 2.
      [1913 Webster]

   Syn: Grave; formal; ritual; ceremonial; sober; serious;
        reverential; devotional; devout. See Grave.
        [1913 Webster]

WordNet (r) 3.0 (2006):

solemn
    adj 1: dignified and somber in manner or character and committed
           to keeping promises; "a grave God-fearing man"; "a quiet
           sedate nature"; "as sober as a judge"; "a solemn
           promise"; "the judge was solemn as he pronounced
           sentence" [syn: grave, sedate, sober, solemn]
    2: characterized by a firm and humorless belief in the validity
       of your opinions; "both sides were deeply in earnest, even
       passionate"; "an entirely sincere and cruel tyrant"; "a film
       with a solemn social message" [syn: earnest, sincere,
       solemn]

Moby Thesaurus II by Grady Ward, 1.0:

213 Moby Thesaurus words for "solemn":
   Christian, Christianlike, Christianly, admiring, adorant, adoring,
   apotheosizing, arid, aristocratic, august, awe-inspiring, awed,
   awesome, awestricken, awestruck, barren, believing, black, blah,
   blank, bleak, bloated, bloodless, bombastic, celebrating,
   celebrative, ceremonial, ceremonious, characterless, cold,
   colorless, commemorating, commemorative, conventional, courtly,
   cultish, cultist, cultistic, dark, dead, decorous, deifying,
   demure, devoted, devotional, devout, dignified, dismal, divine,
   draggy, drear, drearisome, dreary, dry, dryasdust, dull, dusty,
   dutiful, earnest, ecclesiastical, effete, elephantine, elevated,
   empty, etiolated, fade, faithful, fatal, fateful, flat, flatulent,
   formal, formidable, frowning, full, funebrial, funereal, gassy,
   gloomy, glum, grand, grandiloquent, grave, gray, grim, grim-faced,
   grim-visaged, hallowed, heavy, hero-worshiping, hieratic, ho-hum,
   hollow, holy, idolatrous, idolizing, imploring, important,
   imposing, impressive, in awe, in the dust, inane, inexcitable,
   inflated, insipid, inspiring, jejune, kingly, leaden, lifeless,
   liturgic, liturgical, lofty, long-faced, lordly, low-spirited,
   magisterial, magnificent, majestic, memorial, mirthless, moderate,
   momentous, morbid, morose, moving, no-nonsense, noble,
   on bended knee, ostentatious, overwhelming, pale, pallid,
   pedestrian, pietistic, pious, plenary, plodding, pointless, poky,
   pompous, ponderous, pontifical, portentous, prayerful, precative,
   precatory, princely, prostrate before, queenly, regal, religious,
   reserved, reverent, reverential, ritual, ritualistic, royal,
   sacerdotal, sacramental, sacred, saturnine, sedate, self-important,
   serious, slow, sober, sober-minded, sobersided, somber, sombrous,
   spiritless, staid, stately, statuesque, sterile, stiff, stilted,
   stodgy, stone-faced, straight-faced, stuffy, sublime, superficial,
   suppliant, supplicant, supplicatory, swollen, taciturn, tasteless,
   tedious, temperate, theistic, thoughtful, triste, tumid, turgid,
   unlively, unsmiling, vapid, venerable, venerational, venerative,
   weariful, wearisome, weary, weighty, well-mannered, wooden,
   worshipful, worshiping, worthy