marmot, squirrel
鼫 centers on small, rodent-like mammals that live in burrows or trees: specifically a marmot or squirrel. The two meanings are closely related as similar animals.
鼫 is a phono-semantic compound: the left component 鼠 (rat, mouse) indicates the meaning of a rodent-like animal, while the right component 石 likely contributes the sound. The exact historical development is uncertain, but the structure is typical for animal names.
A mouse-like creature (鼠) that lives among rocks (石): picture a marmot or squirrel scurrying over stones.
For セキ, imagine a squirrel sitting on a rock (石, セキ) and squeaking: the rock's reading セキ helps you remember the kanji's reading セキ.