how, indeed, I dare say
嘸 centers on expressing strong conjecture or certainty about a situation: 'how (it must be)', 'indeed', or 'I dare say'. It conveys a sense of inferring or emphasizing what is likely true.
嘸 combines 口 (mouth) with 無, which likely contributes the sound. The character is used to express emphatic conjecture or exclamation, but the exact historical development is uncertain.
A mouth 口 speaking with 無 (nothingness) beside it: imagine someone saying 'how could it be nothing?' with strong emphasis, as if they dare say it must be something.
For ブ, picture a boo of surprise when you realize how indeed something is true: boo -> ブ, and your mouth exclaims the conjecture.