becoming emaciated, son
悴 centers on a state of severe physical or emotional depletion: becoming emaciated, worn down, or haggard. The meaning "son" is a humble, self-deprecating term for one's own son, reflecting a sense of unworthiness or meagerness.
悴 combines 忄 (heart/mind) with 卒, which likely contributes the sound and may suggest suddenness or ending. The exact historical development is uncertain, but the character expresses a heart/mind worn down to a depleted state.
The heart radical 忄 beside 卒 (which can suggest a soldier or sudden end) paints a picture of a heart that has been through hardship and become thin and worn out, like a person who is emaciated. For the "son" meaning, imagine a humble father referring to his son as his own worn-out heart, a self-deprecating term of endearment.
For スイ, imagine a person so emaciated they can only sip water through a straw: sip -> スイ, and the heart 忄 is too weak to do more.