"dish accessory to the principal one in a course," 1725, from side (adj.) + dish (n.). The restaurant phrase on the side "apart from the main dish" is attested from 1884, in American English.
Entries linking to side-dish
side adj.
late Old English, "long, broad, spacious; extending lengthwise," from side (n.). Compare Old Norse siðr "long, hanging down." From late 14c. as "being from or toward the side," hence also "subordinate." Also "apart from the main course" of anything, as in side-road (1854); side-trip (1911). In side-eye (by 1922) the notion is "directed sideways."
dish n.
Old English disc "plate, bowl, platter," from Latin discus "dish, platter, quoit," in Medieval Latin "a table, dais, desk, pulpit," from Greek diskos "disk, platter" (see disk (n.)).
A common West Germanic borrowing; Old High German took the word as tisc "plate," but German Tisch now means "table," in common with some other later Romanic forms of Latin discus (such as Italian desco, French dais); compare desk (n.), dais.
Meaning "particular variety of food served in a dish" is first recorded mid-15c. Meaning "what one likes, what is suited to one's taste" is by 1918; that of "attractive woman" is 1920s. Meaning "concave reflector or antenna" attested from 1948.
Originally applied to very shallow or flat vessels, as plates and platters, the term now usually includes any large open vessel, more or less deep, and with or without a cover, used to contain food or table-drink such as tea, coffee, or chocolate. The use of the term to include drinking-vessels, as bowls and cups, is less common, and seems to be obsolescent, except as such vessels are included in the collective plural dishes. [Century Dictionary, 1897]
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.