plainyoudaoicibaDictYouDict[plain 词源字典]
plain: [13] Plain is etymologically the same word as plane in all its uses except the tree-name, and even that comes from the same ultimate source. This was Indo-European *plā- ‘flat’, which produced Greek platús ‘broad’ (source of English place, plaice, and platypus), Latin plānus ‘flat, clear’, and possibly English flat. The Latin word passed into English via Old French plain, but its original ‘flat’ senses have been hived off into the separately acquired plane, leaving only the metaphorically derived ‘clear’ senses. The Italian descendant of plānus has given English piano.
=> plane[plain etymology, plain origin, 英语词源]
plain (adj.)youdaoicibaDictYouDict
c. 1300, "flat, smooth," from Old French plain "flat, smooth, even" (12c.), from Latin planus "flat, even, level" (see plane (n.1)). Sense of "evident" is from, c. 1300; that of "free from obstruction" is early 14c.; meaning "simple, sincere, ordinary" is recorded from late 14c., especially of dress, "unembellished, without decoration."

In reference to the dress and speech of Quakers, it is recorded from 1824; of Amish and Mennonites, from 1894 (in the Dutch regions of Pennsylvania Plain with the capital is shorthand adjective for "Amish and Old Order Mennonite"). Of appearance, as a euphemism for "ill-favored, ugly" it dates from 1749. Of envelopes from 1913. As an adverb from early 14c. Plain English is from c. 1500. Plain dealer "one who deals plainly or speaks candidly" is from 1570s, marked "Now rare" in OED 2nd edition. To be as plain as the nose on (one's) face is from 1690s.
plain (n.)youdaoicibaDictYouDict
"level country," c. 1300 (in reference to Salisbury Plain), from Old French plain "open countryside," from Latin planum "level ground, plain," noun use of neuter of planus (adj.) "flat, even, level" (see plane (n.1)). Latin planum was used for "level ground" but much more common was campus.