"Mhm" is something we say unconsciously all the time. According to some historians, "mhm" wasn't always used by English speakers. In the U.S., enslaved people introduced it into Southern vernacular. While linguists disagree over whether or not "mhm" is a Black linguistic norm, it wouldn't be the first instance of our ancestry hiding in our language.
John Rickford, a linguistics professor at Stanford University, says that enslavers feared being plotted against, so they forced enslaved people to speak only English.
Our people had the genius to preserve bits of their native languages by masking African words that could pass for English. Words like goober, okra, and banjo come from West Africa, and we brought them to the Americas. The "mhm" sound could have a similar story.
Although the Black roots of “mhm” are open to debate, it's undeniably a part of the culture. We add inflection and gesture to tweak the meaning, from expressing understanding (mhm) to throwing shade (mmmhm).
Black people have always shaped the languages we speak. We've long imbued sounds and words with unique meanings, and it's a testament to our powerful ability to take the languages we were forced to adopt and make them ours.