The Collaborative International Dictionary of English v.0.48:

Plod \Plod\, v. i. [imp. & p. p. Plodded; p. pr. & vb. n.
   Plodding.] [Gf. Gael. plod a clod, a pool; also, to strike
   or pelt with a clod or clods.]
   1. To travel slowly but steadily; to trudge. --Shak.
      [1913 Webster]

   2. To toil; to drudge; especially, to study laboriously and
      patiently. "Plodding schoolmen." --Drayton.
      [1913 Webster]