paralysis, palsy, numbness
癱 centers on a loss of physical function: paralysis, palsy, or numbness where a body part cannot move or feel.
癱 combines 疒 (sickness) with 難, which likely contributes the sound and may suggest difficulty or obstruction, fitting the idea of paralysis.
The sickness enclosure 疒 wraps around 難 (difficulty). Picture a body trapped by illness, making movement difficult or impossible.
For タン, imagine a person paralyzed and unable to get a tan because they cannot move outside: tan -> タン, and the sickness keeps them still.