respectful reply of assent to superiors
喏 centers on a respectful verbal response, specifically an assent or acknowledgment given to a superior. It is a formal, deferential way of saying 'yes'.
喏 combines 口 (mouth) with 若, which likely contributes a phonetic element. The character is used for a respectful vocal reply, fitting the mouth radical.
The mouth 口 speaks the word 若 (which can mean 'young' or 'if'), but here imagine a young subordinate using their mouth to give a respectful 'yes' to an elder.
For シャ, picture a respectful attendant giving a sharp 'sha!' as a crisp, formal acknowledgment. The sound 'sha' cues シャ, and the mouth 口 delivers the reply.