againyoudaoicibaDictYouDict[again 词源字典]
again: [OE] The underlying etymological sense of again is ‘in a direct line with, facing’, hence ‘opposite’ and ‘in the opposite direction, back’ (its original meaning in Old English). It comes from a probable Germanic *gagin ‘straight’, which was the source of many compounds formed with on or in in various Germanic languages, such as Old Saxon angegin and Old Norse íg gegn.

The Old English form was ongēan, which would have produced ayen in modern English; however, Norse-influenced forms with a hard g had spread over the whole country from northern areas by the 16th century. The meaning ‘once more, anew’ did not develop until the late 14th century. From Old English times until the late 16th century a prefix-less form gain was used in forming compounds.

It carried a range of meanings, from ‘against’ to ‘in return’, but today survives only in gainsay. The notion of ‘opposition’ is carried through in against, which was formed in the 12th century from again and what was originally the genitive suffix -es, as in always and nowadays. The parasitic -t first appeared in the 14th century.
[again etymology, again origin, 英语词源]
again (adv.)youdaoicibaDictYouDict
late Old English agan, from earlier ongean "toward, opposite, against, in exchange for," from on "on" (see on) + -gegn "against, toward," compounded for a sense of "lined up facing, opposite," and "in the opposite direction, returning." For -gegn, compare Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to."

In Old English, eft was the main word for "again" (see eftsoons), but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. Differentiated from against 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted).