Etymology
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Mennonite (n.)

member of a German Anabaptist sect, 1560s, from name of Menno Simons (1492-1559), founder of the sect in Friesland and chief exponent of its doctrines (adult baptism, refusal of oaths, civic offices, and support of the state in war), + -ite (1). As an adjective by 1727. Alternative form Mennonist (n.) is attested from 1640s.

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Amish (adj.)
1844, American English, from the name of Jacob Amman, 17c. Swiss Mennonite preacher who founded the sect. The surname is a contraction of Old High German ambahtman, title of an official in the German Swiss cantons, from ambet "office" (German Amt; see amt), a Celtic borrowing related to the beginning of ambassador) + man "man." Originally also spelled Omish, which reflects the pronunciation in Pennsylvania German dialect. As a noun, by 1884. Other early names for the sect were Ammanite and, in a European context, Upland Mennonite.
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plain (adj.)

c. 1300, "flat, smooth," from Old French plain "flat, smooth, even" (12c.), from Latin planus "flat, even, level" (from PIE root *pele- (2) "flat; to spread"). Sense of "explicit, clear, evident" is from late 14c.; that of "free from obstruction" is mid-14c.; meaning "simple, sincere, ordinary" is recorded from late 14c., especially of dress, "unembellished, without decoration, unadorned." Of words, speech, etc., "direct and to the point," late 14c. As an adverb from late 14c.

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.

Plain English is from c. 1500. Plain dealer "one who speaks his opinions candidly; one who is frank, honest, and open" is from 1570s, marked "Now rare" in OED 2nd edition, though it survives since 1842 as the name of the main newspaper of Cleveland, Ohio. To be as plain as the nose on (one's) face "obvious" is from 1690s.

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