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]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "plod":
   amble, andante, barge, bowl along, bundle, chug on, claudication,
   clump, crawl, creep, dead march, dig, dogtrot, drag, drudge, fag,
   flounce, flounder, foot, footpace, footslog, funeral march, grind,
   grub, halt, hammer, hammer away, hippety-hop, hitch, hobble, hop,
   jog, jog on, jog trot, jolt, jump, keep doggedly at, labor,
   leisurely gait, limp, lumber, lumbering pace, lunge, lurch, mince,
   mincing steps, moil, muck, pace, paddle, peg, peg away, piaffe,
   piaffer, plod along, plug, plug along, plug at, plug away,
   pound away, prance, rack, roll, rub on, sashay, saunter, schlep,
   scuff, scuffle, scuttle, shamble, shuffle, sidle, single-foot,
   skip, slave, slink, slither, slog, slog away, slop, slouch,
   slow march, slow motion, stagger, stalk, stamp, stodge, stomp,
   straddle, straggle, stride, stroll, strut, stump, swagger, swing,
   tittup, toddle, toil, totter, traipse, tramp, trample, trash,
   travail, tread, trip, trudge, waddle, wade through, walk, wallow,
   wamble, wiggle, wobble, work, work away