lined (kimono)
袷 centers on clothing that is lined, specifically a lined kimono. The idea is a garment with an inner layer added for structure or warmth.
袷 combines 衤 (clothing) with 合, which likely contributes both sound and the idea of joining or combining, suggesting a garment with layers joined together.
The clothing radical 衤 beside 合 (join) creates a picture of a garment where two layers are joined: a lined kimono.
For コウ, imagine a coat with a warm inner lining: coat -> コウ, and the lined kimono keeps you cozy.