From (the stem of) Latin Pyrrhō + -ism, after Middle French pyrrhonisme.
Pyrrhonism (countable and uncountable, plural Pyrrhonisms)
- The system of skeptical philosophy established by Pyrrho of Elis, centred on the idea that nothing can be known for certain; widespread skepticism, universal doubt. [from 17th c.]
1603, Michel de Montaigne, chapter 12, in John Florio, transl., The Essayes, […], book II, printed at London: By Val[entine] Simmes for Edward Blount […], OCLC 946730821:
Whosoever shall imagine a perpetuall confession of ignorance, and a judgement upright and without staggering, to what occasion soever may chance; That man conceives the true Phyrrhonisme.