How did 秘 "bi" become "mi"? I came across this comment just now:


  “秘”声符是“必”没问题，但是并不是说“秘”和“必”一定同音，“秘”这个字是北京话读错了，其实应该读bi，“秘鲁”一词还残存bi的读音


Which got me thinking.



秘 is still read "bi" in:


surnames
transcriptions (e.g.: Peru)




Cantonese carries the pronunciation:


  bei3




Sichuanese also has 秘 as "bei" in certain instances:


  秘密 beimi 秘，读“背”。成都人通常称有所隐蔽，不让人知道的话及事情：～档案│～活动。又为“背密”。




Matthews' also has it listed under "bi":


  




Middle Chinese pronunciations also seem to be way close to "bi" than "mi."

So what gives? Where did "mi" come from?

edit: I'm thinking maybe 秘 sounded like 屄, and it was changed purposely because of taboos. (http://www.sohu.com/a/219931086_413427) "山东方言对女性的某个器官的发音为“比”，因此，为了避免与比字同音，凡是在读笔字时都要读成北，于是，便有了钢北，毛北，圆柱北。"
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