Ideogrammic compound (會意): 禾 (“grain”) + 火 (“fire”) - burning crops with fire.
In autumn, the Ancient Chinese would burn the fields to kill the locust eggs, in order to prevent them from eating the crops the following year.
From Old Japanese. Likely cognate with 明き (aki, “bright”), 赤 (aka, “red”), 飽きる (akiru, “to become full up”, possibly in reference to the harvest), 空く (aku, “to become empty”), 開く (aku, “to open up”). However, the exact relationship of these terms remains unclear.