Etymology
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side effect (n.)
also side-effect, 1884, from side (adj.) + effect (n.). Medical use, with reference to drugs, is recorded from 1939.
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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, 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.

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side (v.)
late 15c., "to cut into sides" (of meat), from side (n.). Meaning "to support one of the parties in a discussion, dispute, etc.," is first attested 1590s, from side (n.) in the figurative sense; earlier to hold sides (late 15c.). Related: Sided; siding.
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effect (n.)
Origin and meaning of effect
mid-14c., "execution or completion (of an act)," from Old French efet (13c., Modern French effet) "result, execution, completion, ending," from Latin effectus "accomplishment, performance," from past participle stem of efficere "work out, accomplish," from assimilated form of ex "out" (see ex-) + combining form of facere "to make, to do" (from PIE root *dhe- "to set, put"). From French, borrowed into Dutch, German, Scandinavian.

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.
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effect (v.)
Origin and meaning of effect
"to produce as a result; to bring to a desired end," 1580s, from Latin effectus, past participle of efficere "work out, accomplish" (see effect (n.)). Related: Effecting; effection; effectible.
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side-dish (n.)
1725, from side (adj.) + dish (n.). Restaurant phrase on the side "apart from the main dish" is attested from 1884, American English.
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side-swipe (v.)
also sideswipe, "to strike with a glancing blow," 1904, from side (adj.) + swipe (v.). Related: Side-swiped; side-swiping. The noun is first recorded 1917.
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blind side (n.)
"weak or unguarded aspect of a person or thing," c. 1600; see blind (adj.). As a verb, also blindside, "to hit from the blind side," first attested 1968, American English, in reference to U.S. football tackles.
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