The Collaborative International Dictionary of English v.0.48:

Plodding \Plod"ding\, a.
   Progressing in a slow, toilsome manner; characterized by
   laborious diligence; as, a plodding peddler; a plodding
   student; a man of plodding habits. --Plod"ding*ly, adv.
   [1913 Webster]

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]

WordNet (r) 3.0 (2006):

plodding
    adj 1: (of movement) slow and laborious; "leaden steps" [syn:
           leaden, plodding]
    n 1: hard monotonous routine work [syn: drudgery, plodding,
         grind, donkeywork]
    2: the act of walking with a slow heavy gait; "I could recognize
       his plod anywhere" [syn: plodding, plod]

Moby Thesaurus II by Grady Ward, 1.0:

181 Moby Thesaurus words for "plodding":
   application, arid, assiduity, assiduous, assiduousness, banausic,
   barren, blah, blank, bloodless, bulldog tenacity, characterless,
   cold, colorless, concentration, constancy, constant, continuing,
   dead, diligence, diligent, dim, dismal, dogged,
   dogged perseverance, doggedness, draggy, drearisome, dreary,
   drudging, dry, dryasdust, dull, dusty, effete, elephantine, empty,
   endurance, enduring, engrossment, etiolated, fade, faithful,
   fidelity, flat, grinding, grubbing, hardworking, heavy, ho-hum,
   hollow, humdrum, immutable, inalterable, inane, indefatigability,
   indefatigable, indomitable, industrious, industriousness, industry,
   inexcitable, insipid, insistence, insistency, insistent,
   invincible, jejune, laboring, lasting, leaden, lifeless,
   low-spirited, loyal, loyalty, monotone, monotonous, never-tiring,
   obstinacy, obstinate, pale, pallid, patience, patience of Job,
   patient, patient as Job, pedestrian, pegging, permanence,
   permanent, perseverance, perseverant, persevering, persistence,
   persistency, persistent, persisting, pertinacious,
   pertinaciousness, pertinacity, plugging, pointless, poky,
   ponderous, preoccupation, preoccupied, rapt, relentless,
   relentlessness, resolute, resolution, sedulity, sedulous,
   sedulousness, single-minded, single-mindedness,
   singleness of purpose, slaving, sleepless, slogging, slow, solemn,
   spiritless, stability, stable, stamina, staying power, steadfast,
   steadfastness, steadiness, steady, sterile, stick-to-itiveness,
   stiff, stodgy, straining, striving, struggling, stubborn,
   stubbornness, stuffy, superficial, sweating, tasteless, tedious,
   tenacious, tenaciousness, tenacity, tireless, tirelessness,
   toiling, unabating, unconquerable, undaunted, undiscouraged,
   undrooping, unfailing, unfaltering, unflagging, unflinching,
   unintermitting, uninterrupted, unlively, unnodding, unrelaxing,
   unrelenting, unremitting, unremittingness, unsleeping, unswerving,
   unswerving attention, untiring, unwavering, unwearied, unwearying,
   unwinking, utterly attentive, vapid, weariless, wooden, working