noun, adverb
Archaic expression meaning 'from now on' or 'in the future'. Used in classical or formal historical contexts; in modern Japanese, 今後 (こんご) or これから is used instead.
自今以後、この地を守るべし。
Henceforth, you shall protect this land.
Compound of 自 (from), 今 (now), 以 (since), and 後 (after). The exact historical derivation is uncertain, but it follows a classical Chinese pattern for expressing 'from this point forward'.