spread out, smiling, laughing
肸 centers on a sense of spreading or expanding outward, which extends naturally to the idea of a smile or laughter spreading across a face.
肸 is a rare character whose historical development is uncertain. The modern form visually contains 月 (flesh) and a right side that may be related to 十, but the exact origin is unclear.
The left side 月 suggests a body part, and the right side looks like something spreading out from a central point. Imagine a smile spreading across a face, starting from the cheeks (flesh) and widening outward.
For キツ, imagine a kitsune (fox) spreading a wide, mischievous grin: kitsune -> キツ, and the fox's smile spreads across its face.