simple (adj.)

c. 1200, "free from duplicity, upright, guileless; blameless, innocently harmless," also "ignorant, uneducated; unsophisticated; simple-minded, foolish," from Old French simple (12c.) "plain, decent; friendly, sweet; naive, foolish, stupid," hence "wretched, miserable," from Latin simplus from PIE compound *sm-plo-, from root *sem- (1) "one; as one, together with" + *-plo- "-fold."

Sense of "free from pride, humble, meek" is mid-13c. As "consisting of only one substance or ingredient" (opposite of composite or compounded) it dates from late 14c.; as "easily done" (opposite of complicated) it dates from late 15c.

From mid-14c. as "unqualified; mere; sheer;" also "clear, straightforward; easily understood." From late 14c. as "single, individual; whole." From late 14c. of clothing, etc., "modest, plain, unadorned," and of food, "plain, not sumptuous." In medicine, of fractures, etc., "lacking complications," late 14c. As a law term, "lacking additional legal stipulations, unlimited," from mid-14c.

In Middle English with wider senses than recently, such as "inadequate, insufficient; weak, feeble; mere; few; sad, downcast; mournful; of little value; low in price; impoverished, destitute;" of hair, "straight, not curly." As noun, "an innocent or a guileless person; a humble or modest person" (late 14c.), also "an uncompounded substance." From c. 1500 as "ignorant people."

Definitions of simple
simple (adj.)
apart from anything else; without additions or modifications;
the simple truth
the simple passage of time was enough
Synonyms: bare / mere
simple (adj.)
having few parts; not complex or complicated or involved;
simple mechanisms
a simple problem
a simple design
a simple substance
simple (adj.)
easy and not involved or complicated;
a simple game
Synonyms: elementary / uncomplicated / unproblematic
simple (adj.)
exhibiting childlike simplicity and credulity;
Synonyms: childlike / wide-eyed / round-eyed / dewy-eyed
simple (adj.)
lacking mental capacity and subtlety;
Synonyms: dim-witted / simple-minded
simple (adj.)
(botany) of leaf shapes; of leaves having no divisions or subdivisions;
Synonyms: unsubdivided
simple (adj.)
her black dress--simple to austerity
a simple country schoolhouse
simple (n.)
any herbaceous plant having medicinal properties;
simple (n.)
a person lacking intelligence or common sense;
Synonyms: simpleton