deceive, cheat, arrogant
訑 centers on dishonest or self-important speech: deceiving or cheating with words, and speaking with arrogance.
訑 combines 言 (speech) with 也, which likely contributes the sound. The exact historical development of the meaning is uncertain.
Words 言 that twist like a snake 也: imagine a forked tongue speaking deceitfully and arrogantly.
For シ, picture a deceitful person hissing 'Shh!' as they lie: Shh -> シ, and the sound comes from the speaking radical 言.