"the side or border of a road," 1744, from road (n.) + side (n.). As an adjective by 1810.
Entries linking to roadside
road n.
Middle English rode, from Old English rad "riding expedition, journey, hostile incursion," from Proto-Germanic *raido (source also of Old Frisian red "ride," Old Saxon reda, Middle Dutch rede, Old High German reita "foray, raid"), from PIE *reidh- "to ride" (see ride (v.)). Also related to raid (n.).
In Middle English it was still, "a riding, a journey on horseback; a mounted raid;" the sense of "an open passage or way for traveling between two places" is recorded from 1590s, and the older senses now are obsolete. "The late appearance of this sense makes its development from sense 1 somewhat obscure," according to OED, which however finds similar evolutions in Flemish and Frisian words. The modern spelling was established 18c.
The meaning "narrow stretch of sheltered water near shore where ships can lie at anchor" is from early 14c. (as in Virginia's Hampton Roads). In late 19c. U.S. use it is often short for railroad.
On the road "traveling" is from 1640s. Road test (n.) of a vehicle's performance is by 1906; as a verb from 1937. Road hog "one who is objectionable on the road" [OED] is attested from 1886; road rage is by 1988. Road map is from 1786; road trip is by 1950, originally of baseball teams. Old English had radwerig "weary of traveling."
side n.
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, "flank; side (of meat); coast," 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).
The "long part of anything" sense is preserved hillside, it also was in 16c.-17c. side-coat "long coat." From 14c. as "lateral half of the body of a slaughtered animal." In reference to bacon, it indicates position relative to the ribs. The meaning "a region, district" is from c. 1400, as in South Side, countryside.
The figurative sense of "position or attitude of a person or set of persons in relation to another" (as in choose sides, side of the story) is recorded by mid-13c. As "an aspect" of anything immaterial (the bright side, etc.), by mid-15c.
The meaning "one of the parties in a transaction" is from late 14c.. The sense of "one of the parties in a sporting contest or game" is from 1690s. The meaning "music on one side of a phonograph record" is attested by 1936. As short for side-dish, by 1848.
The phrase side by side "close together and abreast, placed with sides near together" is recorded from c. 1200. Colloquial on the side "in addition," especially "unacknowledged," with connotations of "illicit, shady," is by 1893.