Etymology
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siding (n.)
c. 1600, "a taking of sides in a conflict or debate," verbal noun from side. First attested 1825 in the railroad sense; 1829, American English, in the architectural sense of "boarding on the sides of a building."
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pro-slavery (adj.)

"favoring slavery; siding with the political interests of slaveholders," 1825, from pro- + slavery.

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sidetrack (n.)
also side-track, "railway siding," 1835, from side (adj.) + track (n.). The verb meaning "to move (a train car) onto a sidetrack" is from 1874; figurative sense of "to divert from the main purpose" is attested from 1881. Related: Sidetracked.
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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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