stubborn, perverse, aggressive
忮 centers on a defiant, unyielding attitude: stubbornly resisting others, acting perversely, or behaving aggressively.
忮 combines the heart radical 忄 with 支, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with stubborn or perverse attitudes.
A heart 忄 that supports 支 only its own way: imagine a stubborn heart that refuses to bend, leading to perverse and aggressive behavior.
For シ, picture a stubborn person hissing 'Shh!' at any advice: Shh -> シ, and their heart 忄 stays defiant.