Where does “Mahometanism” come from?

Mahometanism derives from English Mahometan plus Ancient Greek -ισμός, which combines Ancient Greek -μός and Proto-Indo-European -mós, a suffix forming abstract nouns and denoting action or condition.

Ancestry of “Mahometanism”, step by step

Mahometanism traces back through two separate lines of ancestry.

via English ism

StepLanguageWordMeaning
1EnglishismAn ideology, system of thought, or practice that...
2English-ismUsed to form nouns of action or process or result...
3Latin-ismus-ism
4Ancient Greek-ισμόςForms abstract nouns
5Ancient Greek-μόςForms abstract nouns
6Proto-Indo-European-mósCreates action/result nouns from verb stems

via English Mahometan

StepLanguageWordMeaning
1EnglishMahometanAlternative form of Mohammedan
Every word from Proto-Indo-European -mósEvery word from English -ismEvery word from English ism