The Collaborative International Dictionary of English v.0.48:

Groan \Groan\, v. t.
   To affect by groans.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Groan \Groan\, n.
   A low, moaning sound; usually, a deep, mournful sound uttered
   in pain or great distress; sometimes, an expression of strong
   disapprobation; as, the remark was received with groans.
   [1913 Webster]

         Such groans of roaring wind and rain.    --Shak.
   [1913 Webster]

         The wretched animal heaved forth such groans. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

groan \groan\ (gr[=o]n), v. i. [imp. & p. p. Groaned
   (gr[=o]nd); p. pr. & vb. n. Groaning.] [OE. gronen, granen,
   granien, AS. gr[=a]nian, fr. the root of grennian to grin.
   [root]35. See 2d Grin, and cf. Grunt.]
   1. To give forth a low, moaning sound in breathing; to utter
      a groan, as in pain, in sorrow, or in derision; to moan.
      [1913 Webster]

            For we . . . do groan, being burdened. --2 Cor. v.
                                                  4.
      [1913 Webster]

            He heard the groaning of the oak.     --Sir W.
                                                  Scott.
      [1913 Webster]

   2. To strive after earnestly, as with groans.
      [1913 Webster]

            Nothing but holy, pure, and clear,
            Or that which groaneth to be so.      --Herbert.
      [1913 Webster]

WordNet (r) 3.0 (2006):

groan
    n 1: an utterance expressing pain or disapproval [syn: groan,
         moan]
    v 1: indicate pain, discomfort, or displeasure; "The students
         groaned when the professor got out the exam booklets"; "The
         ancient door soughed when opened" [syn: groan, moan]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "groan":
   air a grievance, bawl, beef, beefing, belch, bellyache,
   bellyaching, bitch, bitching, blare, blat, bray, burr, buzz,
   cackle, caw, chirr, clamor, clang, clangor, clank, clash, complain,
   complaining, complaint, crab, craunch, croak, crump, crunch, cry,
   cry out, destructive criticism, dissent, dolorous tirade,
   faultfinding, fret, fret and fume, fuss, grievance, grind, gripe,
   griping, groaning, grouch, grouse, grousing, growl, grumble,
   grumbling, grunt, holler, howl, jangle, jar, jeremiad, keen, kick,
   kicking, lament, lodge a complaint, make an outcry, moan, murmur,
   murmuring, mutter, muttering, object, outcry, peeve, peevishness,
   pet peeve, petulance, pipe, plaint, planctus, protest,
   querulousness, raise a howl, rasp, register a complaint, roar,
   scolding, scranch, scrape, scratch, scream, screech, scrunch,
   shriek, sigh, sing, snarl, sniping, snore, sob, sough, squall,
   squawk, squawking, take on, tirade, twang, ululate, ululation,
   wail, wail of woe, whimper, whine, whining, whisper, whistle,
   yammer, yammering, yap, yapping, yawl, yawp, yell, yelp, yowl