Etymology
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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). The baseball diamond sense is from 1867. Baseline estimate was in use by 1983.

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bench-warmer (n.)

1892, baseball slang; see bench (n.) in the sporting sense.

The days for "bench-warmers" with salaries are also past. [New York Sporting News, Jan. 9, 1892]

Old English had bencsittend "one who sits on a bench."

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single (v.)

"to separate from the herd" (originally in hunting, often with forth or out), "select individually from among a number," 1570s, from single (adj.). The baseball sense of "make a one-base hit" is from 1899 (from the noun meaning "one-base hit," which is attested from 1858). Related: Singled; singling.

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whiff (n.)

13c., weffe "foul scent or odor," of imitative origin. Modern form became popular late 16c. with tobacco smoking, probably influenced by whiffle "blow in gusts or puffs" (1560s). The verb in the baseball slang sense "to swing at a ball and miss" first recorded 1913.

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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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flat-footed (adj.)

c. 1600, "with flat feet;" see flat (adj.) + foot (n.). Meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial adverbial use it meant "straightforwardly, downright, resolute" (1828), from notion of "standing firmly."

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tag (n.2)

"children's game," 1738 (in reference to "Queen Mary's reign"), perhaps a variation of Scottish tig "touch, tap" (1721), probably an alteration of Middle English tek "touch, tap" (see tick (n.2)). Baseball sense is from 1912. It's not an acronym and doesn't stand for anything.

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fan (v.)

late Old English fannian "to winnow (grain)," from the noun (see fan (n.1)). Meaning "to stir up air" is from early 15c. Baseball sense of "strike out (a batter)" is by 1909. Related: Fanned; fanning. To fan out "spread out like a hand-held fan," is from 1590s.

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dugout (n.)

also dug-out, 1722, "primitive type of canoe," consisting of a log with the interior hollowed out, American English, from past participle of dig (v.) + out (adv.). Baseball sense is recorded by 1914, from earlier meaning "rough shelter excavated in the side of a bluff or bank" (1855).

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inning (n.)

Old English innung "a taking in, a putting in," gerundive of innian "get within, put or bring in; lodge; include; fill up, restore," from inn (adv.) "in" (see in). Meaning "a team's turn in action in a game" first recorded 1735, usually plural in cricket, singular in baseball.

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