Started as 'facing back' in space before flipping into 'once more' in time — same root as 'against.'
Once more; another time; anew.
From Old English 'ongēan' (towards, opposite, back, again), from 'on-' (on, toward) + '-gēan' (direct, straight), from Proto-Germanic '*gagina' (against, towards, back), from PIE *gʰi- (to go) or *ghe- (to release, let go). The original sense was spatial — 'in the opposite direction, back' — and the temporal sense 'once more' developed from the idea of returning. Cognate with German 'gegen' (against, toward), Old Norse 'gegn' (straight, direct). Key roots: *gagina (Proto-Germanic: "against, towards, back").
'Again' and 'against' are the same word at different stages of development. 'Against' is simply 'again' with an adverbial '-st' suffix (compare 'amid' → 'amidst,' 'among' → 'amongst'). The original meaning of both was spatial — 'facing toward, in opposition to' — and 'again' developed the temporal sense 'once more' from the idea of turning back. The rare verb 'gainsay' (to contradict) preserves the old 'gain-' form, meaning 'to say against.'