suffer, become emaciated, haggard
忰 centers on a state of physical or mental decline: suffering, wasting away, or becoming haggard and emaciated.
忰 combines the heart radical 忄 with 卆, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with suffering and wasting away.
The heart radical 忄 beside 卆 suggests a heart that is breaking down or wasting away. Picture a person whose heart is so burdened that they become haggard and emaciated.
For スイ, imagine a person suffering so much they can only sip water through a straw: sip -> スイ, and their body wastes away.