One possible explanation of using the word "polecat" to label a skunk is its similarity to the French word "pulent," which translates into "stinking." Usage of the term dates back to the New England colonies in the 1680s.
The word "polecat" may have originated from the French word for chicken which is "poulet," referring to the fact that these wild relatives of the ferret would eat chickens. A polecat is a member of the weasel family which has a scientific name of "Mustelidae." The common or European polecat is similar to skunks in that it has a foul odor. The European polecat is also called a foul marten. Martens are also a member of the Mustelidae family.
Skunks, on the other hand, belong to the Mephitidae family. Another possible origin of the word "polecat," the French word for "stinking" could also explain why the term is used to refer to the foul-smelling creatures.