"characterized by a single part," 1590s, from Latin simplex "single, simple, plain, unmixed, uncompounded," literally "onefold," from PIE root *sem- "one, together" (cognates: Latin semper "always," literally "once for all;" Sanskrit sam "together;" see same) + *plac- "-fold," from PIE *plek- "to plait" (see ply (v.1.)). The noun is attested from 1892, "simple uncompounded word."