Let's Play: The Mana Series - Final Fantasy Adventure #004
The word 'indefatigable' is very strange. A little archaic but not exactly middle English or anything. No one uses it. It's obscure and clunky.
I admire words like this because there is a certain personality type that is triggered by them. The shame and frustration they experience when they encounter a word they don't understand (why would anyone know a word like this) manifests as bitter opposition to the linguistic offender.
The reaction is predictable and mildly amusing. More importantly, it can be useful to lull these people into a sense of comfort by presenting them with plain language and embedding one of these trigger words deep within. It has to be discreet enough that they won't be able to spot it through scanning.
Once the trap has been set you need only sit back and wait. Cue the Michael Jackson popcorn GIF. They will blunder into the landmine and go to pieces. At a bare minimum there will be dark accusations of attempts to sound intelligent. Best case scenario, the entire matter will be completely derailed by an obsessive fixation on these pointless yet bizarre words. Words that could easily be substituted with something more contemporary and commonly understood.
But where's the fun and glory in that?