"to move or go sideways," 1690s, back-formation from obsolete Middle English sidlyng (adv.) "obliquely, sideways; aslant; laterally" (early 14c., perhaps in Old English), from side (n.) + adverbial suffix -ling; altered on analogy of verbs ending in -le. Related: Sidled; sidling. Old English had sidlingweg (n.) "sidelong-way, oblique road."
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.