part
誃 centers on separation or parting. Its meaning is narrow and rare, referring to the act of splitting or moving apart.
誃 combines 言 (speech) with 多 (many), but the exact historical development is uncertain. It may have originally expressed parting words or separation through speech, but this is speculative.
Words (言) and many (多) things going separate ways: imagine a conversation where many people part, each going their own direction.
For シ, picture a parting scene where someone says 'see you' (シー・ユー) as they separate: 'see' -> シ, and the words mark the parting.