weak, steep
孱 centers on a state of being physically or structurally weak, lacking strength or firmness. The meaning "steep" is a rare, possibly unrelated sense that does not clearly connect to weakness.
孱 combines 尸 (body) with 孨, which likely contributes both sound and the idea of weakness or frailty. The exact historical development is uncertain.
Under the 尸 (body) enclosure, three 子 (child) shapes are huddled together, suggesting a weak, frail body that needs support.
For セン, imagine a frail body so weak it can only send a faint signal for help: send -> セン.