The story
From attorney (“any personal representative”) and fact (“act, deed”). Both words now rarely have those senses (outside of fixed expressions like this).
Etymology from Wiktionary, CC BY-SA 4.0.
attorney-in-fact
Today it means: An agent of the person giving them the power of attorney (for a specific purpose or for general purposes) to act on his or her behalf. The attorney-in-fact’s power and responsibilities depend on the specific powers granted in the power of attorney document. Full definition →