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]

WordNet (r) 3.0 (2006):

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