WordNet (r) 3.0 (2006):

knothole
    n 1: a hole in a board where a knot came out

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "knothole":
   air hole, armhole, blowhole, bullet-hole, bunghole, cringle,
   deadeye, eye, eyelet, gasket, grommet, guide, keyhole, loop,
   loophole, manhole, mousehole, peephole, pigeonhole, pinhole,
   placket, placket hole, porthole, punch-hole, spiracle, tap, vent,
   venthole