-arian

1.
a suffix forming personal nouns corresponding to Latin adjectives ending in -ārius or English adjectives or nouns ending in -ary, (librarian; proletarian; Rotarian; seminarian; sexagenarian; veterinarian); subsequently productive in English with other Latinate stems, forming nouns denoting a person who supports, advocates, or practices a doctrine, theory, or set of principles associated with the base word: authoritarian; establishmentarian; totalitarian; vegetarian .
Origin
< Latin -āri(us) or -ary + -an
British Dictionary definitions for -arian

-arian

suffix
1.
indicating a person or thing that advocates, believes, or is associated with something: vegetarian, millenarian, librarian
Word Origin
from Latin -ārius-ary + -an