The Collaborative International Dictionary of English v.0.48:

Quintain \Quin"tain\, n. [F. quintaine, LL. quintana; cf. W.
   chwintan a kind of hymeneal game.]
   An object to be tilted at; -- called also quintel. [Written
   also quintin.]
   [1913 Webster]

   Note: A common form in the Middle Ages was an upright post,
         on the top of which turned a crosspiece, having on one
         end a broad board, and on the other a sand bag. The
         endeavor was to strike the board with the lance while
         riding under, and get away without being hit by the
         sand bag. "But a quintain, a mere lifeless block."
         --Shak.
         [1913 Webster]