weasel
鼪 centers on a small, slender carnivorous mammal: the weasel. The meaning is specific and does not extend into broader abstract ideas.
鼪 combines the rodent radical 鼠 with 生, which likely provides the sound. The character is a specific zoological term for a weasel-like animal.
The left side 鼠 suggests a small rodent-like creature, and the right side 生 (life) can remind you of a lively, quick animal. Together they point to a weasel: a small, energetic predator.
For セイ, imagine a weasel saying 'say!' in a high-pitched voice: 'say' -> セイ, as the weasel darts around chattering.