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.
From late 14c. as "power or capacity to produce an intended result; efficacy, effectiveness," and in astrology, "operation or action (of a heavenly body) on human affairs; influence." Also "that which follows from something else; a consequence, a result." From early 15c. as "intended result, purpose, object, intent." Also formerly with a sense of "reality, fact," hence in effect (late 14c.), originally "in fact, actually, really." Meaning "impression produced on the beholder" is from 1736. Sense in stage effect, sound effect, etc. first recorded 1881.