be worn-out, be haggard
顇 centers on a state of extreme physical or mental exhaustion: being worn-out, haggard, or drained of vitality.
顇 combines 卒 (soldier, end, sudden) with 頁 (head, page), suggesting a head or face that has reached an end or is suddenly worn-out. The exact historical development is uncertain.
A soldier (卒) with a haggard face (頁) after a long march: the head shows the exhaustion of being worn-out.
For スイ, imagine a worn-out soldier swaying (スイ) on his feet, his haggard face drooping: sway -> スイ.