Here is a "History of a Musslelman family" from Lancaster County, PA who say the following on their web site: "One theory supported only by family stories is that the Musselman surname is derived from the word Meuse as in River Meuse, which runs between Holland and Germany. Meus-el-man i.e. the man from the River Meuse gradually changed into Meusselman then to Musselman. In our family Bible, it is written Muffelman with a 1799 entry. The double "ff" is an old style writing of "ss". Research indicates a name change occured when leaving the old world."
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