"in, in the midst of," early 12c., from Old English onmang, in late Old English sometimes amang, a contraction of ongemang "among, during," from phrase on gemang, literally "in the crowd or company (of)," from on (see a- (1)) + gemengan "to mingle," from Proto-Germanic *mangjan "to knead together," which is perhaps from a nasalized form of PIE root *mag- "to knead, fashion, fit." The collective prefix ge- was dropped 12c. leaving onmong, amang, among. Compare Old Saxon angimang "among, amid;" Old Frisian mong "among."
<a href="https://www.etymonline.com/word/mong">Etymology of mong by etymonline</a>
Harper, D. (n.d.). Etymology of mong. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mong