decline, wane, weaken
衰 centers on a gradual loss of strength or vitality: declining, waning, or weakening over time.
衰 originally depicted a raincoat made of straw or grass, which weakens and declines with use. The modern form retains the 衣 (clothing) component, but the inner part has been simplified and its exact historical development is uncertain.
Picture a worn-out coat (衣) that is falling apart at the seams. The fraying fabric represents decline and weakening.
For スイ, imagine a swimmer (スイマー) whose strength declines as they tire in the water: swimmer -> スイ, and their energy wanes.
decline; degeneration; decay; waning; ebbing
weakening; emaciation; decline
weakness; debility; breakdown; prostration
attenuation; damping; decay
to become weak; to decline; to wear; to abate; to decay; to wither; to waste away
senility; senile decay; infirmity (through age)
rise and fall; ups and downs; welfare; vicissitudes
ups and downs of life; vicissitudes of fortune
inevitable decline (decay, collapse, etc.)
decline and fall; ruin; downfall; collapse
to become emaciated; to waste away; to grow thin and worn out
echo suppressor
five signs of the impending death of a heavenly being
general prostration
return loss
to waste away
to languish
prosperity and decay; rise and fall; ebb and flow; vicissitudes (of fortune)
attenuation distortion
attenuation coefficient