During the Crusades, the Muslims of the Middle East came to call the invading Western (Latin) Christians Franks, originally the name for inhabitants of the largest of the Latin Christian realms in Europe, Francia, which gave its name to the Kingdom of France (although its eastern parts came to be known as the Holy Roman Empire).
As the Franks (French) formed a substantial part of the force of the First Crusade, and Old French became the dominant language in the crusader states of the 12th century (notably the Principality of Antioch), the term Frank as used in the Levant could mean any Western European (Latin) Christian (whether Frankish, Saxon, Flemish, etc.).
Frangistan was not a clearly defined area and may have referred to any land perceived to be Western European Christian by contemporary Muslims.
Conversely, Christians generally called Muslims Saracens or Moors, both after the names of more localized tribes, in Arabia and Mauretania, respectively.
[2] While in Persia, it remained in use until the end of the Qajar dynasty as observed in various correspondences and administrative documents of that era to refer to European countries.