Yours, Etc.: Origins and Uses of 8 Sign-Offs
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."
Old English side "flanks of a person, the long part or aspect of anything," from Proto-Germanic *sithon (cf. 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" (cf. 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" (cf. 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.
late 14c., from side (n.).