くさる

to rot; to go bad; to decay; to spoil; to fester; to decompose; to turn sour (e.g. milk)

to corrode; to weather; to crumble

to become useless; to blunt; to weaken (from lack of practice)

to become depraved; to be degenerate; to be morally bankrupt; to be corrupt

to be depressed; to be dispirited; to feel discouraged; to feel down

to have the audacity to; to be bastard enough to