出典:Wiktionary
Superficially from dē- + fatīscō (“to split open, tire”), but de Vaan posits that dēfetīscor came first as a direct formation dē- + *fatis (“exhaustion”) + -scō.
dēfetīscor (present infinitive dēfetīscī, perfect active dēfessus sum); third conjugation, deponent