The Collaborative International Dictionary of English v.0.48:

Weir \Weir\ (w[=e]r), Wear \Wear\,n. [OE. wer, AS. wer; akin to
   G. wehr, AS. werian to defend, protect, hinder, G. wehren,
   Goth. warjan; and perhaps to E. wary; or cf. Skr. v[.r] to
   check, hinder. [root]142. Cf. Garret.]
   1. A dam in a river to stop and raise the water, for the
      purpose of conducting it to a mill, forming a fish pond,
      or the like.
      [1913 Webster]

   2. A fence of stakes, brushwood, or the like, set in a
      stream, tideway, or inlet of the sea, for taking fish.
      [1913 Webster]

   3. A long notch with a horizontal edge, as in the top of a
      vertical plate or plank, through which water flows, --
      used in measuring the quantity of flowing water.
      [1913 Webster]

WordNet (r) 3.0 (2006):

weir
    n 1: a low dam built across a stream to raise its level or
         divert its flow
    2: a fence or wattle built across a stream to catch or retain
       fish

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "weir":
   aboideau, air lock, arch dam, avenue, backstop, bamboo curtain,
   bank, bar, barrage, barrier, bear-trap dam, beaver dam, blowhole,
   boom, breakwater, breastwork, brick wall, buffer, bulkhead,
   bulwark, channel, chute, cofferdam, dam, debouch, defense, dike,
   ditch, dock gate, door, earthwork, egress, embankment, emunctory,
   escape, estuary, exhaust, exit, fence, flood-hatch, floodgate,
   flume, gate, gravity dam, groin, head gate, hydraulic-fill dam,
   iron curtain, jam, jetty, leaping weir, levee, lock, lock gate,
   logjam, loophole, milldam, moat, mole, mound, opening, out,
   outcome, outfall, outgate, outgo, outlet, parapet, penstock, pore,
   port, portcullis, rampart, roadblock, rock-fill dam, sally port,
   seawall, shutter dam, sluice, sluice gate, spiracle, spout,
   stone wall, tap, tide gate, vent, ventage, venthole, vomitory,
   wall, water gate, way out, wicket dam, work



U.S. Gazetteer Places (2000):

Weir, KS -- U.S. city in Kansas
   Population (2000):    780
   Housing Units (2000): 352
   Land area (2000):     1.046565 sq. miles (2.710591 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    1.046565 sq. miles (2.710591 sq. km)
   FIPS code:            76350
   Located within:       Kansas (KS), FIPS 20
   Location:             37.308768 N, 94.774289 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Weir, KS
    Weir


U.S. Gazetteer Places (2000):

Weir, TX -- U.S. city in Texas
   Population (2000):    591
   Housing Units (2000): 229
   Land area (2000):     1.593171 sq. miles (4.126293 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    1.593171 sq. miles (4.126293 sq. km)
   FIPS code:            77056
   Located within:       Texas (TX), FIPS 48
   Location:             30.675007 N, 97.587862 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Weir, TX
    Weir


U.S. Gazetteer Places (2000):

Weir, MS -- U.S. town in Mississippi
   Population (2000):    553
   Housing Units (2000): 234
   Land area (2000):     1.050416 sq. miles (2.720566 sq. km)
   Water area (2000):    0.011427 sq. miles (0.029595 sq. km)
   Total area (2000):    1.061843 sq. miles (2.750161 sq. km)
   FIPS code:            78520
   Located within:       Mississippi (MS), FIPS 28
   Location:             33.263423 N, 89.289439 W
   ZIP Codes (1990):     39772
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Weir, MS
    Weir