Beside

google

ref

Old English be sīdan (adverb) ‘by the side’ (see by, side).

wiktionary

ref

From Middle English beside, besiden, bisyde (also besides > besides), from Old English be sīdan, bī sīdan(“by the side (of), on the side (of)”), equivalent to be- +‎ side. Compare Saterland Frisian biesiede(“aside”), German Low German bisied(“aside”), German beiseite(“aside, to one side”). Compare also Dutch terzijde(“aside”).

etymonline

ref

beside (prep., adv.)

c. 1200, from Old English be sidan "by the side of" (only as two words), from be- + sidan dative of side (n.). By 1200 as one word and used as both adverb and preposition. The alternative Middle English meaning "outside" is preserved in beside oneself "out of one's wits" (late 15c.).