'Foumart' is the Middle English and modern Scots word for a polecat (for us Americans, a polecat is a species of weasel). It is specifically the European polecat, Mustela putorius. Foumart literally meant "foul martin", 'foul', in this case, referring to the strong scent that they give off when threatened.
Foumart was used idiomatically in much the same way as we use weasel today, although there were positive connotations; if you called a person a foumart you probably meant that he was in some way offensive, but it might also mean that they were quick-witted. Foumart-faced meant exactly the same as weasel-faced means today. Foumartish, on the other hand, was nothing like 'weaselly', instead meaning 'strong-smelling'.
While the term foumart was used to refer to polecats in general, in practice it usually meant the Scottish subspecies, known formally as the Scottish polecat, M. p. caledoniae. This species was viewed as a pest because it killed poultry, and a bounty had been offered for foumart corpses since the 1300s. Later, the foumart became commercially important as a source of furs, a market that peaked and then collapsed in the mid-1800. The last surviving Scottish polecat was recorded in 1914, and they are now believed to be extinct. In 2004 Scottish polecats were again found in the wild, although it is currently believed that this is probably not M. p. caledoniae, but some other subspecies that has been reintroduced into Scotland.