Etymology
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bottom line (n.)

figurative sense is attested from 1832, from profit-and-loss accounting, where the final figure calculated is the bottom line on the page. Also (especially as an adjective) bottom-line, bottomline.

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plumb-line (n.)

"a cord or line with a metal bob attached to one end, used to determine vertical direction," mid-15c., from plumb (n.) + line (n.).

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bee-line (n.)

also beeline, "straightest line between two points," 1830, American English, from bee + line (n.), in reference to the homing of bees in the field.

TO LINE BEES is to track wild bees to their homes in the woods. One who follows this occupation is called a bee hunter. [Bartlett, 1859]

The verbal phrase line bees is attested from 1827.

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clothes-line (n.)
also clothesline, 1830, from clothes + line (n.). As a kind of high tackle in U.S. football (the effect is similar to running into a taut clothesline) attested by 1970; as a verb in this sense by 1959.
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front-line (n.)
also frontline, 1842 in the military sense, from front (adj.) (1520s, from front (n.)) + line (n.). As an adjective from 1915.
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main-top (n.)

also maintop, "top of the mainmast," late 15c.; see mainmast + top (n.1). By 1725 as "platform just below the head of the mainmast."

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Maginot Line 

1936, network of defensive fortifications built along the northern and eastern borders of France before World War II, in which the French placed unreasonable confidence, named for André Maginot (1877-1932), French Minister of War under several governments in the late 1920s and early 1930s. After the fall of France in 1940, for the next 40 years or so the phrase was associated with a mental attitude of obsessive reliance on defense.

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life-line (n.)
also lifeline, 1700, "rope used to save lives" in any way (especially for the safety of sailors on vessels in bad weather or on the yards), from life (n.) + line (n.); figurative sense first attested 1860. Sense in palmistry from 1890.
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line-up (n.)
also lineup, from the verbal phrase line up (1889 as "form a line;" 1902 as "make into a line"); see line (v.2) + up (adv.). As a noun, the baseball version (1889) is older than the police version (1907).
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hard-line (adj.)
"uncompromising," 1958, originally in reference to Soviet communist policies, from the noun phrase (see hard (adj.) + line (n.)) in the political sense. Related: Hard-liner (1963).
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