The Collaborative International Dictionary of English v.0.48:

Cucking stool \Cuck"ing stool`\ (k?k"?ng st??l`). [Cf. AS.
   scealfingst[=o]l, a word of similar meaning, allied to
   scealfor a diver, mergus avis; or possibly from F. coquine a
   hussy, slut, jade, f. of coquin, OE. cokin, a rascal; or cf.
   Icel. k?ka to dung, k?kr dung, the name being given as to a
   disgracing or infamous punishment.]
   A kind of chair formerly used for punishing scolds, and also
   dishonest tradesmen, by fastening them in it, usually in
   front of their doors, to be pelted and hooted at by the mob,
   but sometimes to be taken to the water and ducked; -- called
   also a castigatory, a tumbrel, and a trebuchet; and
   often, but not so correctly, a ducking stool. --Sir. W.
   Scott.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ducking \Duck"ing\,
   n. & a., from Duck, v. t. & i.
   [1913 Webster]

   Ducking stool, a stool or chair in which common scolds were
      formerly tied, and plunged into water, as a punishment.
      See Cucking stool. The practice of ducking began in the
      latter part of the 15th century, and prevailed until the
      early part of the 18th, and occasionally as late as the
      19th century. --Blackstone. Chambers.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Castigatory \Cas"ti*ga*to*ry\, n.
   An instrument formerly used to punish and correct arrant
   scolds; -- called also a ducking stool, or trebucket.
   --Blacktone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ducking stool
    n 1: an instrument of punishment consisting of a chair in which
         offenders were ducked in water [syn: cucking stool,
         ducking stool]