Also in M-W, I noticed "katharevusa". I do not anticipate
having any use for it.
Katharevusa

SYLLABICATION: Ka·tha·rev·u·sa
PRONUNCIATION: käthä-rv-sä
NOUN: The puristic, archaizing form of Modern Greek, having morphological and lexical features borrowed from Koine.
ETYMOLOGY: Modern Greek kathareuousa, from Greek, feminine present participle of kathareuein, to be pure, from katharos, pure.
I do remember that there was a Christian heresy with
similar name in southern France in Middle Ages, viciously
exterminated.