Brumbaugh
A surname most likely derived from a German place-name ending in -bach, later reshaped in America.
The #6,635 most common surname in the United States (2010 Census) — about 1 in every 60,503 Americans shares it.
Etymology
The name is generally treated as an Americanized form of a German surname built on a place-name with -bach, meaning "brook" or "stream." The historical material you provided links Brumbaugh with forms such as Bumbach and compares it with other surnames ending in -baugh that reflect German -bach. The first element is less certain, but the overall sense is likely "the brook" or a brook-associated place name.
History
The excerpted scholarship suggests Brumbaugh belongs to the large set of German-American surnames that changed in spelling after immigration, especially those ending in -bach becoming -baugh. The name is established in the United States and is notably associated with Pennsylvania German and other Mid-Atlantic German-speaking communities. A 20th-century bearer mentioned in your source list is M. G. Brumbaugh.
Origin
Regions
Variant spellings
Fun fact
In old German-American surname studies, Brumbaugh is often cited as a textbook example of how -bach could shift to -baugh in English spelling.
From the historical record
“C. J. HEXAMER, ARNO LEONHARDT, M. G. BRUMBAUGH, STUART CULIN, M. D. LEARNED, Committee.”
— Studies in Pennsylvania German Family Names