英语词源字典 - 在线词源网站

plain (adj.)[plain 词源字典]: 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 etymology, plain origin, 英语词源]
seal (v.): "to fasten with (or as with) a seal," c. 1200, from seal (n.1). Meaning "to place a seal on (a document)" is recorded from mid-14c.; hence "to conclude, ratify, render official" (late 15c.). Sense of "to close up with wax, lead, cement, etc." is attested from 1660s, from the notion of wax seals on envelopes. In reference to the actions of wood-coatings, 1940. Related: Sealed; sealing. Sealing-wax is attested from c. 1300. To seal (one's) fate (1799) probably reflects the notion of a seal on an execution warrant.