The Collaborative International Dictionary of English v.0.48:

Yawn \Yawn\ (y[add]n), v. i. [imp. & p. p. Yawned; p. pr. &
   vb. n. Yawning.] [OE. yanien, [yogh]anien, ganien, gonien,
   AS. g[=a]nian; akin to ginian to yawn, g[imac]nan to yawn,
   open wide, G. g[aum]hnen to yawn, OHG. gin[=e]n, gein[=o]n,
   Icel. g[imac]na to yawn, gin the mouth, OSlav. zijati to
   yawn, L. hiare to gape, yawn; and perhaps to E. begin, cf.
   Gr. cheia` a hole. [root]47b. Cf. Begin, Gin to begin,
   Hiatus.]
   [1913 Webster]
   1. To open the mouth involuntarily through drowsiness,
      dullness, or fatigue; to gape; to oscitate. "The lazy,
      yawning drone." --Shak.
      [1913 Webster]

            And while above he spends his breath,
            The yawning audience nod beneath.     --Trumbull.
      [1913 Webster]

   2. To open wide; to gape, as if to allow the entrance or exit
      of anything.
      [1913 Webster]

            't is now the very witching time of night,
            When churchyards yawn.                --Shak.
      [1913 Webster]

   3. To open the mouth, or to gape, through surprise or
      bewilderment. --Shak.
      [1913 Webster]

   4. To be eager; to desire to swallow anything; to express
      desire by yawning; as, to yawn for fat livings. "One long,
      yawning gaze." --Landor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

yawning
    adj 1: gaping open as if threatening to engulf someone or
           something; "the yawning mine shaft"; "a yawning abyss"
    2: with the mouth wide open indicating boredom or sleepiness; "a
       yawning congregation"
    3: showing lack of attention or boredom; "the yawning
       congregation" [syn: drowsy, oscitant, yawning(a)]
    n 1: an involuntary intake of breath through a wide open mouth;
         usually triggered by fatigue or boredom; "he could not
         suppress a yawn"; "the yawning in the audience told him it
         was time to stop"; "he apologized for his oscitancy" [syn:
         yawn, yawning, oscitance, oscitancy]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "yawning":
   abysmal, abyssal, agape, ajar, anesthetized, bottomless,
   cataleptic, cavernous, comatose, deep as hell, dehiscence,
   dehiscent, doped, doziness, dozy, dreamy, drowsiness, drowsy,
   drugged, drugged with sleep, fathomless, gape, gaping, half asleep,
   heaviness, heavy, heavy with sleep, heavy-eyed, in a stupor,
   languid, languor, lethargic, lethargy, napping, narcoleptic,
   narcose, narcotized, narcous, nodding, openmouthed, oscitancy,
   oscitant, oscitation, out of it, pandiculation, plumbless,
   plunging, ringent, sedated, slack-jawed, sleep-drowned,
   sleep-drunk, sleep-filled, sleep-swollen, sleepful, sleepiness,
   sleepy, slumberous, slumbery, snoozy, somnolence, somnolency,
   somnolent, soporific, soundless, stretching, stretchy, stuporose,
   stuporous, the gapes, unfathomable, unfathomed, unsounded,
   without bottom, yawn, yawny