Etymology
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mainstream (n.)
also main-stream, main stream, "principal current of a river," 1660s, from main (adj.) + stream (n.); hence, "prevailing direction in opinion, popular taste, etc.," a figurative use first attested in Carlyle (1831). Mainstream media attested by 1980 in language of U.S. leftists critical of coverage of national affairs.
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waterline (n.)
also water-line, 1620s, line where the water rises to on the hull of a ship afloat, from water (n.1) + line (n.).
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hemline (n.)
also hem-line, 1899, from hem (n.) + line (n.).
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baseline (n.)

also base-line, "line upon which others depend," 1750, originally in surveying, from base (n.) + line (n.). In tennis, the end-line of the court (1872). Baseball diamond sense is from 1867. Baseline estimate in use by 1983.

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rectilinear (adj.)

1650s, "forming a straight line," with -ar + rectiline (1560s), from Late Latin rectilineus, from rectus "straight" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line") + linea "line" (see line (n.)). Of a figure, "bounded by straight lines," 1728. Related: Rectilineal "straight-lined" (1640s); rectilinearity.

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waistline (n.)
also waist-line, 1867, from waist + line (n.).
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shoreline (n.)
also shore-line, 1852 in the geographical sense, from shore (n.) + line (n.).
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mainstay (n.)

"chief support," 1787, a figurative use of a nautical noun meaning "stay which extends from the main-top to the foot of the foremast" (late 15c.), from main (adj.) + stay (n.).

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neckline (n.)

also neck-line, "shape of the top of a woman's garment at the front," 1900, from neck (n.) + line (n.).

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mainmast (n.)

also main-mast, "the tallest mast in a sailing ship," late 15c., from main (adj.) + mast (n.1). In three-masted vessels, the middle mast; in four-masted ships the second from the bow.

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