[Verdex_3]

The GURPS role playing system uses the term indefatigable as part of its game mechanics. Additionally, the Film Reroll Podcast uses GURPS as their game system. My guess is that OP either has played GURPS or listened to the Film Reroll.

Also OP could just be really well read.

Personally, I'm glad to see indefatigable in the wild (at least more in the wild than a game system and a pod cast). I think the word is really cool and I haven't found an opportunity to use it myself.