Aside

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Middle English (originally on side ): see a2, side.

wiktionary

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From Middle English aside, asyde, a-side, oside, from Middle English on side, from Old English on sīdan(literally “on (the) side (of)”), equivalent to a- +‎ side. Compare beside.

etymonline

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aside (adv.)

c. 1300, "off to one side;" mid-14c., "to or from the side;" late 14c., "away or apart from a normal direction or position, out of the way," from a- (1) "on" + side (n.). Noun sense of "words spoken so as to be (supposed) inaudible" is from 1727. Middle English had asidely "on the side, indirectly" (early 15c.) and asideward "sideways, horizontal" (late 14c.). Used colloquially as a preposition from 1590s.