The prefix de- is not used only in loanwords, you can construct new words with it just fine. It doesn't sound right in your example but that doesn't mean it's nonsense.
> The prefix de- is not used only in loanwords, you can construct new words with it just fine.
Listing some examples would strengthen your argument immensely.
> It doesn't sound right in your example but that doesn't mean it's nonsense.
To my native ear, it sounds nonsensical. I wouldn't be able to guess what it is supposed to mean.
It was the Czech person who first combined the Czech/Latin prefix, the Latin stem and the Czech/Slavic suffix in a decidedly Czech sentence.
> Listing some examples would strengthen your argument immensely.
You said one yourself. Nobody would say it because there's already a better way to say that, but everybody would understand the meaning and the grammar is fine.
> but everybody would understand the meaning and the grammar is fine.
No, I certainly wouldn't. It's nonsense.
Can you name a word formed like that which is present in some dictionary?
(I had the displeasure of studying Latin in school)
Note that all Latin speakers in that time period spoke Latin as their second (third...) language, and it was pretty common to see influence of other (mother) languages onto the used Latin. It wouldn't be surprising if e.g. a German native speaker (where such word concoctions are common place) coined such a Latin word.
Here ( https://www.perseus.tufts.edu/hopper/morph?l=defatigatio&la=... ) is a dictionary entry citing defatigatio to a speech given by Cicero.