Meaning

adverb on or to one side

noun a line spoken by an actor to the audience but not intended for others on the stage

adverb out of the way (especially away from one's thoughts)

originFrom 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.

Etymology from Wiktionary, CC BY-SA 3.0