The Collaborative International Dictionary of English v.0.48:

Abrupt \Ab*rupt"\, n. [L. abruptum.]
   An abrupt place. [Poetic]

         "Over the vast abrupt."                  --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Abrupt \Ab*rupt"\, v. t.
   To tear off or asunder. [Obs.] "Till death abrupts them."
   --Sir T. Browne.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Abrupt \Ab*rupt"\, a. [L. abruptus, p. p. of abrumpere to break
   off; ab + rumpere to break. See Rupture.]
   1. Broken off; very steep, or craggy, as rocks, precipices,
      banks; precipitous; steep; as, abrupt places. "Tumbling
      through ricks abrupt," --Thomson.
      [1913 Webster]

   2. Without notice to prepare the mind for the event; sudden;
      hasty; unceremonious. "The cause of your abrupt
      departure." --Shak.
      [1913 Webster]

   3. Having sudden transitions from one subject to another;
      unconnected.
      [1913 Webster]

            The abrupt style, which hath many breaches. --B.
                                                  Jonson.
      [1913 Webster]

   4. (Bot.) Suddenly terminating, as if cut off. --Gray.
      [1913 Webster]

   Syn: Sudden; unexpected; hasty; rough; curt; unceremonious;
        rugged; blunt; disconnected; broken.
        [1913 Webster]

WordNet (r) 3.0 (2006):

abrupt
    adj 1: marked by sudden changes in subject and sharp
           transitions; "abrupt prose" [syn: abrupt,
           disconnected]
    2: exceedingly sudden and unexpected; "came to an abrupt stop";
       "an abrupt change in the weather"
    3: extremely steep; "an abrupt canyon"; "the precipitous rapids
       of the upper river"; "the precipitous hills of Chinese
       paintings"; "a sharp drop" [syn: abrupt, precipitous,
       sharp]
    4: surprisingly and unceremoniously brusque in manner; "an
       abrupt reply"

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "abrupt":
   aggressive, arduous, bearish, beastly, bluff, blunt, blunt-edged,
   blunt-ended, blunt-pointed, blunted, bluntish, bold, brash,
   breakneck, breathless, brief, brisk, brusque, casual, cavalier,
   churlish, crisp, crusty, curt, discourteous, dull, dull-edged,
   dull-pointed, dulled, dullish, edgeless, electrifying, faired,
   gruff, harsh, hasty, headlong, hurried, impetuous, impolite,
   impulsive, informal, nerve-shattering, obtuse, panting,
   perpendicular, plumb, plunging, pointless, precipitant,
   precipitate, precipitous, quick, rapid, rash, ready, rough,
   rounded, rude, rushing, severe, sharp, sheer, shocking, short,
   sideling, smoothed, snappish, snappy, snippety, snippy, speedy,
   startling, steep, stickle, sudden, surly, surprising, swift,
   truculent, unannounced, unanticipated, unceremonious, uncivil,
   unedged, unexpected, unforeseen, unlooked-for, unplanned,
   unpointed, unpredicted, unsharp, unsharpened, vertical



The Devil's Dictionary (1881-1906):

ABRUPT, adj.  Sudden, without ceremony, like the arrival of a cannon-
shot and the departure of the soldier whose interests are most
affected by it.  Dr. Samuel Johnson beautifully said of another
author's ideas that they were "concatenated without abruption."