say, reason, pretext, history, past, flat sun radical (no. 73)
曰 centers on spoken expression: saying something, giving a reason, or offering a pretext. Its extended uses for history and the past come from the idea of things that have been said or recorded.
曰 is a pictograph of a mouth with a line inside, representing speech or something said. The modern form preserves this basic shape.
The wide open shape looks like a mouth with a tongue or words inside: 曰 is a mouth saying something.
For エツ, imagine an editor (エディター) saying 'yes' to a story: the 'e' from editor and the 'tsu' from 'yes' blend into エツ, as the editor speaks the final word.