cannot, be unable do, improbable, thereupon
叵 centers on impossibility or inability: something that cannot be done or is improbable. The 'thereupon' sense is a rare, context-dependent extension.
叵 is traditionally explained as 可 (possible) reversed, symbolizing the opposite idea: cannot or impossible. The exact historical development is uncertain, but the reversal concept is widely noted.
Imagine the box 匚 trapping a mouth 口, making it impossible to speak. The idea of 'cannot' is locked inside.
For ハ, picture a person laughing 'Ha!' at something impossible: Ha -> ハ, and the trapped mouth cannot speak.