a 'fail-safe' device/mechanism [=a device/mechanism that will not break]
He was looking for a 'fail-safe' source of money.
Of course, no system is entirely 'fail-safe'.
(..) Oh, Sylvester, you disappoint me. Every host's spine, mine included, has an explosive charge which will detonate if I try to leave. Isn't that right? A fail-safe. Which vertebra was it? C. But you'd need a full rebuild to replace it. I'm sure that can be arranged. (..)2016 TvShow: Westworld Title: Trace Decay Season: 1 Episode 8