A navvy was what Charles Dickens derisively called the Ninth Doctor upon their first encounter on Christmas Eve, 1869. Dickens made the comparison on the basis of the Doctor's attire. (TV: The Unquiet Dead)
The Fourth Doctor called Romana II someone with "all the makings of a first-class navvy" after she correctly identified the basic formula for cement and concrete. (TV: Destiny of the Daleks)
Behind the scenes
In real life, a navvy was a manual labourer who was typically engaged in the construction of civil engineering projects. The term was especially in use in 18th century Britain, and chiefly employed with respect to the "navigational engineers" who built Britain's navigational channels, or canals. Thus, Dickens' employment of the term was a class-based insult. Likewise, the Fourth Doctor was clearly trying to put Romana in her place after a particularly diffident display of geologic knowledge.