side (n.) Look up side at Dictionary.com
Old English side "flanks of a person, the long part or aspect of anything," from Proto-Germanic *sithon (cognates: 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.
side (v.) Look up side at Dictionary.com
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.
side (adj.) Look up side at Dictionary.com
late 14c., from side (n.).