The Collaborative International Dictionary of English v.0.48:

Rowlock \Row"lock\ (r[=o]"l[o^]k colloq. r[u^]l"[u^]k), n. [For
   oarlock; AS. [=a]rloc, where the second part is akin to G.
   loch a hole, E. lock a fastening. See Oar, and Lock.]
   1. (Naut.) A contrivance or arrangement serving as a fulcrum
      for an oar in rowing. It consists sometimes of a notch in
      the gunwale of a boat, sometimes of a pair of pins between
      which the oar rests on the edge of the gunwale, sometimes
      of a single pin passing through the oar, or of a metal
      fork or stirrup pivoted in the gunwale and suporting the
      oar; same as oarlock. [Chiefly Brit.]
      [1913 Webster +PJC]

   2. One of the rings of masonry included in an arch having
      more than one ring.
      [PJC]

WordNet (r) 3.0 (2006):

rowlock
    n 1: a holder attached to the gunwale of a boat that holds the
         oar in place and acts as a fulcrum for rowing [syn: peg,
         pin, thole, tholepin, rowlock, oarlock]