checking out, conking (out), croaking(slang), demising, departing, dropping, dying, ending, exiting, expiring, falling, flatlining, going, kicking in(slang), kicking off(slang), parting, passing away, passing (on), pegging out(chiefly British), perishing, popping off, stepping out, succumbing

predeceasingconsuming, disappearing, drying up, fading, failing

biting the dust, buying it(or buying the farm), giving up the ghost, kicking the bucket, snuffing it(British)