The Collaborative International Dictionary of English v.0.48:

Navvy \Nav"vy\, n.; pl. Navies. [Abbreviated fr. navigator.]
   Originally, a laborer on canals for internal navigation;
   hence, a laborer on other public works, as in building
   railroads, embankments, etc. [Eng.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

navvy
    n 1: a laborer who is obliged to do menial work [syn: drudge,
         peon, navvy, galley slave]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "navvy":
   blue-collar worker, breadwinner, casual, casual laborer,
   common laborer, day laborer, digger, dredge, dredger, driller,
   employee, excavator, factory worker, flunky, free lance,
   free-lancer, full-time worker, groundhog, hand, industrial worker,
   jobber, jobholder, laborer, laboring man, menial, migrant, miner,
   moiler, office temporary, proletarian, roustabout, salaried worker,
   sandhog, sapper, self-employed person, servant, steam shovel,
   stiff, temporary, toiler, tunneler, wage earner, wage slave,
   wageworker, worker, workgirl, workhand, working girl, workingman,
   workingwoman, workman