simplicity n.

"characterized by a single part," 1590s, from Latin simplex "single, simple, plain, unmixed, uncompounded," literally "one-fold," from PIE compound of root *sem- (1) "one; as one, together with" + *plac- "-fold," from PIE root *plek- "to plait." Compare duplex.

The noun is attested from 1892 in grammar, "simple uncompounded word," from a sense of the Latin word. Simplex munditis is a Latin phrase implying "elegantly simple."

late 14c., "innocence, blamelessness," also "unity, wholeness," also "absence of pride," from simple (adj.) + -ness. From late 14c. as "absence of duplicity; ignorance; absence of complexity."

Middle English also had simplesse "innocence, blamelessness; lack of pride or ostentation," from the Old French compound. Simplesse is attested in English by late 14c. as "wholeness, unity," by c. 1400 as "ignorance." 

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."