The Collaborative International Dictionary of English v.0.48:

Shibboleth \Shib"bo*leth\, n. [Heb. shibb[=o]leth an ear of
   corn, or a stream, a flood.]
   1. A word which was made the criterion by which to
      distinguish the Ephraimites from the Gileadites. The
      Ephraimites, not being able to pronounce sh, called the
      word sibboleth. See --Judges xii.
      [1913 Webster]

            Without reprieve, adjudged to death,
            For want of well pronouncing shibboleth. --Milton.
      [1913 Webster] Also used in an extended sense.
      [1913 Webster]

            The th, with its twofold value, is . . . the
            shibboleth of foreigners.             --Earle.
      [1913 Webster]

   2. Hence, the criterion, test, or watchword of a party; a
      party cry or pet phrase.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shibboleth
    n 1: a favorite saying of a sect or political group [syn:
         motto, slogan, catchword, shibboleth]
    2: a manner of speaking that is distinctive of a particular
       group of people

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "shibboleth":
   banality, bromide, byword, catch phrase, catchword, cliche,
   countersign, cry, fad word, jargon, open sesame, password,
   pet expression, phrase, platitude, prosaicism, prosaism,
   rubber stamp, secret grip, slogan, tag, tessera, token, truism,
   vogue word, watchword



Easton's 1897 Bible Dictionary:

Shibboleth
   river, or an ear of corn. The tribes living on the east of
   Jordan, separated from their brethren on the west by the deep
   ravines and the rapid river, gradually came to adopt peculiar
   customs, and from mixing largely with the Moabites, Ishmaelites,
   and Ammonites to pronounce certain letters in such a manner as
   to distinguish them from the other tribes. Thus when the
   Ephraimites from the west invaded Gilead, and were defeated by
   the Gileadites under the leadership of Jephthah, and tried to
   escape by the "passages of the Jordan," the Gileadites seized
   the fords and would allow none to pass who could not pronounce
   "shibboleth" with a strong aspirate. This the fugitives were
   unable to do. They said "sibboleth," as the word was pronounced
   by the tribes on the west, and thus they were detected (Judg.
   12:1-6). Forty-two thousand were thus detected, and

     "Without reprieve, adjudged to death,

     For want of well-pronouncing shibboleth."


Hitchcock's Bible Names Dictionary (late 1800's):

Shibboleth, Sibboleth, ear of corn; stream or flood