It’s about 3.5 inches long, tapered square section on the outside, with a thick circular flange at the base.
Internally, it’s got a square-section hole running though it, which turns into a circular hole at the thin end. No internal thread in the circular hole.
It’s made of heavy steel - cast or forged, and appears to be powder coated with blue plastic.
I’m assuming this has fallen off a vehicle - either something driving along the road, or a piece of farm machinery on a trailer.
It looks as though it’s meant to fit over a square shaft that is topped with cylindrical rod that would protrude through the hole - maybe that rod would be threaded and a nut would keep this thing in place.
There’s no significant evidence of rotary wear though - so whatever this thing is, it does not appear to be designed to move once placed.
It is a operator nut for a gate valve. It fits over the brass stem of the valve. The round hole at the end if for a bolt to secure it to the valve stem. The top picture here. Resilient Wedge Gate Valves | Clow Valve