How did 秘 "bi" become "mi"?
This is probably not the answer you're looking for, but both「秘」and「泌」underwent the same kinds of shifts (b &gt; m) in certain topolects, so I would just treat it as a simple phonological shift within the labial consonant group (rather than something more &quot;exciting&quot;). I do not believe that the comments on it being a &quot;Mandarin thing&quot; are correct.


Although &quot;time&quot; seems like a cop-out, I would point to similar shifts over &quot;time&quot; like