Etymology
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side (n.)

Old English side "flanks of a person, the long part or aspect of anything," from Proto-Germanic *sīdō (source also of Old Saxon sida, Old Norse siða, Danish side, Swedish sida, Middle Dutch side, Dutch zidje, Old High German sita, German Seite), from adjective *sithas "long" (source of Old English sid "long, broad, spacious," Old Norse siðr "long, hanging down"), from PIE root *se- "long, late" (see soiree).

Original sense preserved in countryside. Figurative sense of "position or attitude of a person or set of persons in relation to another" (as in choosing sides) first recorded mid-13c. Meaning "one of the parties in a transaction" is from late 14c.; sense in a sporting contest or game is from 1690s. Meaning "music on one side of a phonograph record" is first attested 1936. Phrase side by side "close together and abreast" is recorded from c. 1200. Side-splitting "affecting with compulsive laughter" is attested by 1825.

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side (v.)
late 15c., "to cut into sides" (of meat), from side (n.). Meaning "to support one of the parties in a discussion, dispute, etc.," is first attested 1590s, from side (n.) in the figurative sense; earlier to hold sides (late 15c.). Related: Sided; siding.
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side-dish (n.)
1725, from side (adj.) + dish (n.). Restaurant phrase on the side "apart from the main dish" is attested from 1884, American English.
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side-way (n.)
also sideway, 1550s, lateral space for passage or movement," from side (n.) + way (n.).
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side effect (n.)
also side-effect, 1884, from side (adj.) + effect (n.). Medical use, with reference to drugs, is recorded from 1939.
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side-saddle (n.)
"saddle made for the occupant to ride on with both feet on the same side of the horse," used chiefly by women, late 15c., from side (adj.) + saddle (n.).
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side-swipe (v.)
also sideswipe, "to strike with a glancing blow," 1904, from side (adj.) + swipe (v.). Related: Side-swiped; side-swiping. The noun is first recorded 1917.
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