Would you believe it stands for acre-foot? ☝️🤓

The acre-foot is a non-SI unit of volume equal to about 1,233 m3 commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, irrigation water,[1] and river flows.

Haha. No, I looked this up just 2 minutes ago, but what a coincidence, huh? 🤣