Etymology
Advertisement
header (n.)
"head-first dive or plunge," 1849, from head (n.); as a type of pass or shot with the head in soccer, by 1906. Earlier it meant "executioner, headsman" (mid-15c.).
Related entries & more 
Advertisement
double-header (n.)

1869, American English, in early use a kind of fireworks, also a railway train pulled by two engines (or pulled by one, pushed by the other), 1878; see double (adj.) + head (n.). Baseball sense of "two games between the same teams played in the same place on the same day" is by c. 1890.

Related entries & more 
twi-night (adj.)
1939, in reference to evening double-header baseball games, from twilight + night.
Related entries & more 
split (n.)
1590s, "narrow cleft, crack, fissure," from split (v.). Meaning "piece of wood formed by splitting" is from 1610s. Meaning "an act of separation, a divorce" is from 1729. From 1861 as the name of the acrobatic feat. Meaning "a drink composed of two liquors" is from 1882; that of "sweet dish of sliced fruit with ice cream" is attested from 1920, American English. Slang meaning "share of the take" is from 1889. Meaning "a draw in a double-header" is from 1920.
Related entries & more 
nightcap (n.)

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]
Related entries & more 
Advertisement