Etymology PIE word *bʰréh₂tēr Clipping of brother originating in African-American Vernacular English. Predates, but perhaps influenced, other slang clippings spelled with -h. This is a phonetic clipping: /ˈbɹʌðɚ/ → /bɹʌ/. Contrast bro, which is a clipping of the spelling of brother rather than the pronunciation.
[…] in adolescence, and still more in youth, it is no less certain that the bruh is both good-natured and intelligent.1838, James Rennie, The Natural History of Monkeys, Opossums and Lemurs: