The Collaborative International Dictionary of English v.0.48:

Reef \Reef\, v. t. [imp. & p. p. Reefed (r[=e]ft); p. pr. &
   vb. n. Reefing.] (Naut.)
   To reduce the extent of (as a sail) by rolling or folding a
   certain portion of it and making it fast to the yard or spar.
   --Totten.
   [1913 Webster]

   To reef the paddles, to move the floats of a paddle wheel
      toward its center so that they will not dip so deeply.
      [1913 Webster]