bite
噬 centers on the act of biting, with a focus on the mouth's action of seizing or cutting with the teeth.
噬 combines 口 (mouth) with 筮, which likely contributes the sound. The exact historical development of 筮 is uncertain, but the overall structure suggests a phono-semantic compound related to biting.
The mouth 口 on the left is ready to bite, and the right side 筮 looks like a complex tool or set of teeth. Imagine a mouth clamping down with a sharp, tool-like bite.
For ゼイ, picture a zany animal giving a playful bite: zany -> ゼイ, and the mouth 口 snaps shut.