also night-cap, late 14c., "covering for the head, worn in bed," from night + cap (n.). In the alcoholic sense, it is attested from 1818. American English sense of "final event in a sporting contest" (especially the second game of a baseball double-header) is by 1924.
Sunday's baseball opening brought out New York vs. Cleveland in the first game. with Philadelphia and Cincinnati as the star attraction in the nightcap number. [The Typographical Journal, September 1923]
"one who or that which relieves," late 15c., agent noun from relieve. Baseball sense ("relief pitcher," who replaces the starting pitcher when tired or in difficulty) is attested by 1945.
1909, originally in baseball slang, said to be from common pronunciation of "that's the boy!" a cheer of encouragement or approval. I'm the boy for ______ "I'm willing and capable at" is attested from 1843. Related: Attagirl (1924).
1926, a word from the early days of radio (see blooper). In baseball, "hit a ball in a high arc over the head of a fielder," by 1940. Related: Blooped; blooping. As a noun from 1931.
"cheer, support," 1889, American English, originally in a baseball context, probably from root (v.1) via intermediate sense of "study, work hard" (1856). Related: Rooted; rooting.
1530s, "one who throws violently," agent noun from hurl (v.). From c. 1600 as "one who plays at hurling;" from 1926 in baseball slang as "pitcher."
in baseball, "a low batting average," (somewhere around .200) with the suggestion that any player hitting below it ought to feel a bit ashamed, by 1984, said to have been in humorous use in baseball clubhouses c. 1979, from the name of former Pirate, Mariner, and Ranger shortstop Mario Mendoza, who was noted for his defense but whose .215 lifetime batting average routinely left him at the bottom of weekly batting averages. The surname is Basque.
"to take out of a (baseball) game," 1902, from bench (n.) in the sporting sense. Earlier it meant "to display (a dog) in a dog show" (1863). Related: Benched; benching. Old English had a verb bencian, but it meant "to make benches."