Where does “incendiarist” come from?

incendiarist (English) comes from English incendiary, from Old French incendiaire, from Latin incendiārius, from Latin incendium, from Latin incendō, from Latin īn-, from Latin în, from Latin en — in.

Ancestry of “incendiarist”, step by step

incendiarist traces back through two separate lines of ancestry.

via English incendiary

StepLanguageWordMeaning
1EnglishincendiaryCapable of, or used for, or actually causing...
2Old Frenchincendiaire
3Latinincendiāriuscausing a conflagration, setting on fire
4LatinincendiumA fire, inferno, conflagration; heat; torch;...
5Latinincendōto set on fire, burn, kindle
6Latinīn-un-, non-, not
7Latinînin, at, on, upon, from (space)
8Latinenlookǃ beholdǃ; reallyǃ?; come onǃ
9Proto-Italicenin
10Proto-Indo-Europeanh₁énin

via English ist

StepLanguageWordMeaning
1EnglishistA practitioner or supporter of an ism
2English-istAdded to words to form nouns denoting
3Old French-iste
4Latin-ista-ist; one who practises or believes
5Ancient Greek-ιστήςAlternative form of -τής; -ist
6Ancient Greek-τήςAppended to verbs to form agent nouns
7Proto-Hellenic-tās
Every word from Proto-Indo-European h₁énEvery word from Latin īn-Every word from Proto-Italic en