alsoyoudaoicibaDictYouDict[also 词源字典]
also: [OE] Also was a late Old English compound formed from all ‘exactly, even’ and swa ‘so’; it meant ‘in just this way, thus’, and hence (recalling the meaning of German also ‘therefore’) ‘similarly’. These two uses died out in, respectively, the 15th and 17th centuries, but already by the 13th century ‘similarly’ was developing into the current sense ‘in addition’. As came from also in the 12th century. In Old English, the notion of ‘in addition’ now expressed by also was verbalized as eke.
=> as[also etymology, also origin, 英语词源]
also (adv.)youdaoicibaDictYouDict
Old English eallswa "just as, even as, as if, so as, likewise," compound of all + so. The demonstrative sense of "similarly" weakened to "in addition to" in 12c., replacing eke. The compound has parallel forms in German also, Dutch alzoo.