Etymology
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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."

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simple-minded (adj.)
1744, from simple (adj.) + -minded. Related: Simple-mindedly; simple-mindedness.
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simplism (n.)
"oversimplification," 1955, from simple (adj.) + -ism.
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simpleness (n.)
14c., "absence of pride," from simple (adj.) + -ness. From late 14c. as "absence of duplicity; ignorance; absence of complexity."
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simplistic (adj.)
"simple, plain, not compound," 1844, from simple (adj.) + -istic. From 1867 as "over-simple, trying to explain too much by a single principle." Also (1860) "of or pertaining to simples" (herbs used in healing, medicine of one ingredient only; the notion being that each herb possesses a particular virtue, thus a "simple" remedy), from simplist "one who studies simples" (1590s; see simple (adj.)) + -ic.
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simpleton (n.)
1640s, probably a jocular formation from simple and -ton, suffix extracted from surnames. Compare skimmington, personification of an ill-used spouse, c. 1600.
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simply (adv.)
late 13c., simpleliche; see simple + -ly (2). Purely intensive sense is attested from 1580s.
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haplo- 

before vowels hapl-, word-forming element meaning "simple, single; simply, once," from Greek haploos, haplous "single, simple" (as opposed to "compound"); "natural, plain," from PIE compound *sm-plo-, from root *sem- (1) "one; as one, together with" + *-plo- "-fold" (from PIE root *pel- (2) "to fold"). Compare simple, which represents the same compound in Latin.

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*sem- (1)

Proto-Indo-European root meaning "one; as one, together with."

It forms all or part of: anomalous; anomaly; assemble; assimilate; ensemble; facsimile; fulsome; hamadryad; haplo-; haploid; hendeca-; hendiadys; henotheism; hetero-; heterodox; heterosexual; homeo-; homeopathy; homeostasis; homily; homo- (1) "same, the same, equal, like;" homogenous; homoiousian; homologous; homonym; homophone; homosexual; hyphen; resemble; same; samizdat; samovar; samsara; sangha; Sanskrit; seem; seemly; semper-; sempiternal; similar; simple; simplex; simplicity; simulacrum; simulate; simulation; simultaneous; single; singlet; singular; some; -some (1); -some (2); verisimilitude.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sam "together," samah "even, level, similar, identical;" Avestan hama "similar, the same;" Greek hama "together with, at the same time," homos "one and the same," homios "like, resembling," homalos "even;" Latin similis "like;" Old Irish samail "likeness;" Old Church Slavonic samu "himself."

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simplex (adj.)
"characterized by a single part," 1590s, from Latin simplex "single, simple, plain, unmixed, uncompounded," literally "onefold," from PIE compound of root *sem- (1) "one; as one, together with" + *plac- "-fold," from PIE root *plek- "to plait." The noun is attested from 1892, "simple uncompounded word."
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