winter variant radical (no. 34)
夊 is a rare radical variant associated with winter. Its standalone meaning is unclear, and it primarily serves as a component in other characters.
夊 is a variant form of the winter radical 夂, but the exact historical development and original pictographic meaning are uncertain.
The shape 夊 looks like a stylized foot or a trailing leg, often associated with winter in compound characters. Think of it as a frozen footprint in the snow.
For スイ, imagine a winter scene where you see a swing covered in frost: swing -> スイ, and the cold swing reminds you of the winter radical.