get thin, lose weight, become sterile
瘠 centers on physical wasting: becoming thin, losing weight, or declining into an unproductive, sterile state. The meanings share the idea of a body or land losing its substance and vitality.
瘠 combines 疒 (sickness) with 脊, which likely contributes both sound and the idea of a lean spine or backbone, suggesting a wasting condition.
The sickness enclosure 疒 wraps around 脊, a shape associated with the spine. Picture a sick person whose spine shows through their skin as they get thin and lose weight.
For セキ, imagine a sick person wasting away in a seki (席, seat) at a clinic: seki -> セキ, and the sickness enclosure 疒 reminds you of their thin, sterile condition.
barren land; arid soil