This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 肞 is uncertain. It visually combines the flesh component ⺼ with 叉, but the original meaning and component relationship are unclear.
With no clear meaning, picture the flesh component ⺼ beside 叉 as a fork piercing meat, but remember that this is just a visual image, not a real meaning.
For サ, imagine a chef saying 'sa!' as they pierce the meat with a fork: sa -> サ.