I've heard from someone that 八 was at one point pronounced bia1 in old Mandarin. The topic came up when discussing the term biaji, a word for chewing loudly. I couldn't find any evidence for it, since the -j- is not present in any other Sinitic languages that I know of or in Middle Chinese reconstructions.
3 Answers
I think the word you are looking for is "吧唧(biaji/baiji)", which is pronounced as [bā jī]. One of its meanings is "open and close one's lips that make a loud, disturbing sound while eating". It has nothing to do with 八 except the shared sound [bā].
Hope this is what you need. Actually “Bia Ji” still popular in spoken language, but the standard Chinese uses “Ba Ji” 叭叽。
In some places, such as my hometown Jinhua, 八 was indeed pronounced bia in ancient times. And in Jinhua dialect, the pronunciation of 八 is still bia.