According to The History of Defiance County, Ohio:Containing a history of the county; its townships, towns, etc.; military record; portraits of early settlers and prominent men; farm views, personal reminiscences, etc. [Warner, Beers & Company:Chicago, 1883], p. 362, Jacob's brief biography in this source states:
"Jacob Erman was born in Rockingham County, Va., February 4, 1820.His grandfather, George Erman, was a Hessian, born in Prussia, whose wife was born in Switzerland.He came to America, was married, and settled in York County, Penn., and served in the Revolution under Gen. Washington.George Erman, father of our subject, was born in York County, Penn., and, in 1806, married Sarah Argenbright, a native of Rockingham County, Va.They have ten children--John, Polly, Malinda, Delilah, Ann, Jacob, George, Elijah, Caroline and Amanda Jane.February 1, 1844, Jacob married Lucy A. Churchman, daughter of Thomas and Susan Churchman.She was born in Preble County, Ohio, October 12, 1822.Her grandfather, Thomas Churchman, was born in London, England.Mahlon O. Erman, the only child of Jacob and Lucy Erman, was born April 3, 1848, married Serena J. Purtee, daughter of Capt. John and Nancy [Brown] Purtee, and resides in Section 28, this township.Jacob Erman resides on his farm, Section 29.He came to this country in 1853."
Note that there is a question among researchers whether "grandfather, George Erman" was "a Hessian, born in Prussia."