"pertaining to or suggestive of the Middle Ages," 1825 (mediaeval), coined in English from Latin medium "the middle" (from PIE root *medhyo- "middle") + aevum "age" (from PIE root *aiw- "vital force, life; long life, eternity").
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.
<a href="https://www.etymonline.com/word/medievalism">Etymology of medievalism by etymonline</a>
Harper, D. (n.d.). Etymology of medievalism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/medievalism