Just to add a little to the discussion of the origin of the word "navvy", it was always my impression that the navvy was just a step above the common labourer, mainly because of his special skills in the excavation or building of works requiring a straight line, e.g., canals, roads, bridges, dams, etc. It seems that over the years it has taken on a pejorative meaning, especially when used to refer to Irish workers, but originally it was a term of respect.