vegetarianism (n.)

1839, irregular formation from vegetable (n.) + -arian, as in agrarian, etc. "The general use of the word appears to have been largely due to the formation of the Vegetarian Society in Ramsgate in 1847" [OED]. As an adjective from 1849. An earlier adjective was anti-carnivorous (1828).

word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.