to bare the shoulder
裼 centers on exposing the upper body, specifically baring the shoulder, often by removing or adjusting clothing.
裼 combines 衤 (clothing) with 易, which likely contributes the sound. The character is associated with baring the shoulder, but the exact historical development of the compound is uncertain.
The clothing radical 衤 beside 易 suggests an action involving clothes. Picture pulling aside a garment to bare the shoulder, as if making it easy (易) to expose the skin.
For セキ, imagine baring your shoulder at a seki (席, seat) during a hot day: seki -> セキ, and you loosen your clothing to cool off.