Dictionaries generally write
fail-safe as a hyphenated expression. The American Heritage Dictionary and Wiktionary show
failsafe as an alternative form.
The News on the Web has plenty of examples of both versions: 2176 of
fail-safe and 1665 of
failsafe. A recent example is from a Forbes article about the "Internet of things", on the 22 February this year:
Unfortunately, despite all efforts, when it comes to securing IoT, there are no failsafe solutions. It is a daunting challenge.
And an example from the same date of the word used as a noun, from a
Chicago Tribune article:
Colliton, who had several concussions during his playing career, explained the complexities of dealing with them despite the league’s failsafes.
It will be accepted in future, as a rare word.
