slander, utter evil words
讟 centers on harmful speech: slander and uttering evil words. It conveys the act of speaking maliciously to damage someone's reputation.
讟 is a phono-semantic compound with 言 (speech) on the right and 讀 (read) on the left, which likely contributes the sound and may also hint at reciting or uttering words. The exact historical development is uncertain.
The right side is 言 (words), and the left side is 讀 (read). Imagine someone reading aloud evil words to slander another person.
For トク, picture a person uttering slander that is so toxic it feels like a poison: toxic -> トク, and the evil words spread like venom.