This is an extremely interesting question, and, as you have not yet received an answer, I will take a quick shot at it.

I will assume that your great-grandfather lived in late-17th century MD, or, at the very least, after 1666. The latter is an extremely important date, as the very first legislation concerning Maryland's roads was passed by the General Assembly in April of that year- the "Act for making high wayes...for horse and foote," sometimes referred to as the General Road Law of 1666.

Maryland was still in its infancy in the 1660s, Cecil Calvert (2nd Baron Baltimore, sometimes referred to colloquially as "Lord Baltimore") had only received a royal charter in 1632, and St. Mary's City, the first English settlement in Maryland, was founded two years later. Indeed, Maryland had very few roads in 1666, although some infrastructure existed along the Chesapeake Bay. Most of Central Maryland was still inhabited at this point by the Piscataway Native Americans.

This lack of existing infrastructure, coupled with the lack of heavy machinery during the 17th century meant that constructing roads in the new colony was extremely labor-intensive. Facing the same problems, the colony of Virginia had, thirty years earlier, in 1632, passed in its House of Burgesses, legislation mandating the construction of roads and requiring male residents of the colony to work on these roads for a certain amount of time annually. Those who did not provide the colony with this mandatory labor had to either pay someone else to do the work for them or face a fine. This legislation was (to my understanding) more or less a direct carryover from the contemporary practices back in England.

Using Virginia's legislation as an example, Maryland's General Assembly mandated that "the Commissioners of each County shall once this yeare upon the 20th day of October next ensuing meete together...to consult of what high wayes are most fitt to be made...as...to make passable for horse and foote the heads of Rivers, Creekes, Brances, and Swamps." For the actual construction of such "highwayes," the Assembly empowered the commissioners "to appoint overseers" and "leavy tobacco or labour equally to be assessed upon the taxables of each county."

If the commissioners neglected to perform their duties, they were to be fined "4000 pounds of casked tobacco," any lazy overseers were to be fined "2000 pounds of casked tobacco," and every "laborer who shall refuse or neglect his labour" was to be fined "1000 pounds of casked tobacco."

It is notable that, similarly to Virginia, Maryland's legislature required every resident of whatever County constructed a new road, to pay his share of the work in either tobacco or labor. While such a civic burden would be very much illegal today, the laws of early Virginia and Maryland were based significantly upon existing English customs, with the Constitution and the Bill of Rights still over a hundred years in the future. Other legislation from the period, such as the Maryland Mill Act (1669-1766) actually allowed for the seizure of private land with compensation rates often below market value (a similarly illegal situation based upon the Bill of Rights).

Thus, it is extremely likely your ancestor was among those men who decided to "refuse or neglect his labor," for whatever reason, and, having refused this, was accordingly fined by his County's government.

Sources

Hart, John F. “The Maryland Mill Act, 1669-1766: Economic Policy and the Confiscatory Redistribution of Private Property.” The American Journal of Legal History 39, no. 1 (1995): 1. https://doi.org/10.2307/845747.

McIlwaine, H. R. Journals of the House of Burgesses of Virginia, 1659/60-1693. Richmond, VA: Colonial Press, E. Waddey Co., 1914.

Merrell, James H. “Cultural Continuity among the Piscataway Indians of Colonial Maryland.” The William and Mary Quarterly 36, no. 4 (1979): 548. https://doi.org/10.2307/1925183.

MD State Archives Sources

http://aomol.msa.maryland.gov/000001/000002/pdf/am2--134.pdf http://aomol.msa.maryland.gov/000001/000002/pdf/am2--135.pdf