Simplicity
late Middle English: from Old French simplicite or Latin simplicitas, from simplex (see simplex).
wiktionary
From Middle English simplicite, from Old French simplicite, from Latin simplicitās, from simplex(“simple”). See simple. Partially displaced native English onefoldness.
etymonline
simplicity (n.)
late 14c., "singleness of nature, unity, indivisibility; immutability," from Old French simplicite (12c., Modern French simplicité), from Latin simplicitatem (nominative simplicitas) "state of being simple, frankness, openness, artlessness, candor, directness," from simplex (genitive simplicis) "simple" (see simplex). Sense of "ignorance" is from c. 1400; that of "simplicity of expression, plainness of style" is early 15c.
Middle English also had simplesse, from French, attested in English from mid-14c. in sense "humility, lack of pride," late 14c. as "wholeness, unity;" c. 1400 as "ignorance."
