sidecar (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.

c. 1300, "wheeled vehicle," from Anglo-French carre, Old North French carre, from Vulgar Latin *carra, related to Latin carrum, carrus (plural carra), originally "two-wheeled Celtic war chariot," from Gaulish karros, a Celtic word (compare Old Irish and Welsh carr "cart, wagon," Breton karr "chariot"), from PIE *krsos, from root *kers- "to run." The Celtic-Latin word also made it into Greek, as karron "wagon with four wheels."

"From 16th to 19th c. chiefly poetic, with associations of dignity, solemnity, or splendour ..." [OED]. Used in U.S. by 1826 of railway freight carriages and of passenger coaches on a railway by 1830; by 1862 of streetcars or tramway cars. The extension to "automobile" is by 1896, but between 1831 to the first decade of 20c. the cars meant "railroad train." Car bomb is attested from 1972, in a Northern Ireland context. The Latin word also is the source of Italian and Spanish carro, French char.