Casting the anchor
Let's go back to when men first started sailing.
Back then, anchors were rocks with lines wrapped around them to hold the boat in place. They later evolved to rocks with a hole in it for making sure the rope didn't slip.
These were carried on deck, and when you went to use them you threw them over the side. You would want to throw (or cast) them over the side so as to not have the rock hit the side of the ship and damage it. Hence, casing anchor.
The first fluked anchor stopped the rocks and were hung over the side at the cathead.
Fun looking that up.
Enjoy, my British cousins