Mussaman curry is probably the most 'un-Thai' style of Thai curries. It's more like a stew than other Thai curries. The word mussaman has no meaning in Thai, other than as the name of this curry. It may be, like several words in the modern Thai language, a corruption of a foreign word, probably Persian if the stories around this recipe are to be believed. The legend of the dish's origin is that it is derived from a recipe bought by the first Persian ambassador to the Court of Ayutthaya (the capital of old Siam).
Potatoes are a relatively recent introduction to the Thai table. Although potatoes originated in South America, the Spanish and Portuguese apparently did not bring them to Asia, as they did with many other new world foods such as chilies.
Potatoes on sale in the "hill tribe" market in Chiang Mai.