shallow, stupid
譾 centers on a lack of depth or substance: something shallow, superficial, or intellectually dull. The meaning extends naturally from physical shallowness to mental stupidity.
譾 combines 言 (speech) with 翦, which likely contributes both sound and the idea of cutting or reducing, suggesting speech that is cut short or shallow.
The left side 言 is speech, and the right side 翦 looks like a pair of wings (羽) being cut (前 with a knife). Imagine words that are clipped and shallow, like a bird with trimmed wings that cannot fly deep.
For セイ, picture a shallow person saying (言) only 'say' (セイ) and nothing deeper: 'say' -> セイ, and their speech is as shallow as a puddle.