栄枯盛衰[eikoseisui] ups and downs of life
減衰[gensui] attenuation
減衰器[gensuiki] attenuator
古び衰える[furubiotoroeru] to waste away
弱り衰える[yowariotoroeru] to languish

神経衰弱[shinkeisuijaku] neurasthenia
衰える[otoroeru] to become weak
衰え果てる[otoroehateru] to be utterly spent or crushed
衰運[suiun] declining fortunes
衰躯[suiku] emaciated body