shallow, stupid
謭 centers on a lack of depth or substance: something shallow, superficial, or foolish. The 'stupid' sense follows naturally from shallowness of thought.
謭 combines 言 (speech) with 剪, which likely contributes the sound and may relate to cutting or reducing, suggesting speech that is cut short or lacking depth. The exact historical development is uncertain.
Words (言) that are cut short (剪) like a shallow stream: speech that lacks depth, leading to foolishness.
For セイ, imagine a shallow person saying (言) only 'say' (セイ) without deeper thought: 'say' -> セイ, and their words are shallow.