bad-mouth, criticize, defects
訾 centers on pointing out flaws through speech: bad-mouthing, criticizing, or focusing on defects. The idea is verbal fault-finding.
訾 combines 此 (this, here) and 言 (speech). The exact historical development is uncertain, but the character likely uses 此 for its sound and 言 to indicate verbal criticism or fault-finding.
The top part 此 looks like a pointing finger, and the bottom 言 is speech. Imagine someone pointing out defects with sharp words: that's bad-mouthing and criticizing.
For シ, picture a critic pointing at a defect and saying 'See?': 'See' -> シ, as they bad-mouth the flaw.