far be it!: a flat, horrified refusal that rules an act out as unthinkable
This is not a polite "no thank you." It is a cry that shoves a suggested act away as something that must never happen, and a speaker almost always says it before giving any reason. It lands at turning points: Joseph's brothers protesting their innocence, Samuel refusing to stop praying for a people who have just rejected him, Naboth refusing to hand over his family's land, David refusing to lift a hand against Saul.
The word is usually traced back to the verb for treating something holy as common (חָלַל), so the refusal carries a shudder in it. What is being proposed would not merely be unwise, it would be a desecration. That is why the word clusters around acts that would break faith with God, with a king, or with someone the speaker owes loyalty to.
Its force does not change from one use to the next; only the person being cleared does. Whether a speaker is clearing himself, clearing a friend, or clearing God, the word does the same work: it declares the act named to be out of the question. That is why it can stand entirely alone, with nothing attached to it, and still be understood (1 Samuel 14:45), and why Joab can simply say it twice in a row when he wants it to land harder (2 Samuel 20:20).