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]

The Collaborative International Dictionary of English v.0.48:

Plod \Plod\, v. t.
   To walk on slowly or heavily.
   [1913 Webster]

         The ploughman homeward plods his weary way. --Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plod
    n 1: the act of walking with a slow heavy gait; "I could
         recognize his plod anywhere" [syn: plodding, plod]
    v 1: walk heavily and firmly, as when weary, or through mud;
         "Mules plodded in a circle around a grindstone" [syn:
         slog, footslog, plod, trudge, pad, tramp]