What is this?
 

It is about 12×12 at the top with a concave surface. It also weighs aabout 100lbs.

A footing?

Just a WAG on my part.

Near the front porch, something to step on to clean your boots. Prolly homemade by a blacksmith 150 years ago. If the top is connected to the base, check the quality of the welding, or lack thereof, underneath.

Looks like it may be some sort of post cap, support?

Looks like an old cast iron mooring bollard ...like you would weld on or set in cement.

Yep... I think Fint and Bob nailed it.

Would a mooring bollard have a concave top that would serve no purpose other than becoming a pan of water after every rain? Looks like that top is designed for a purpose to me....

I dunno :D

get that surface torch hot and sear some meat on it!

The "top" is probably the bottom.

I don't think so. No anchor points to secure it and bollard would have a domed top so the line can slide down. Top is concave like a cradle. It would be interesting to see the otherside.

Hawaii? Could be related to pineapple or sugar cane processing apparatus. Or military or nautical. But since, I assume, this is Oahu my guess would be military scrap. It looks like it could have been part of a support member for a large diameter pipe, culvert?

Looks like the end of a propeller shaft support strut for a large ship.

Best
Les

Flip it over.

It could be anything, but bollards are almost any size/shape imaginable as they are often purpose built. They are often anchored/set in (and sometimes filled with) mortar or welded down.

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"As the runner stone moves across the surface of the bedstone the kernels of grain are carried up the incline to where the feather edges meet the lands."

Wouldn't there be some mounting holes if it was a mooring bollard??

Yep, I know what a bollard is and that they come in many shapes and sizes. If you look at the OP photo again and read the description this thing is 12"x12" and weighs about 100lbs. Since Brian states it weighs about 100lbs., I would assume he lifted it off the ground. So whatever this thing is that is it.. There is nothing to sink into the ground. I don't see any holes so no way to fasten it to the ground. Note all the photos of bollards you posted have round tops and the OP photo shows a square. A bollard would not have a square as that could snag the eye in a line. The concave top would also allow a line to rest as opposed to side over.

I would not be surprised to see the underside hollowed out to fit over something like a post. The concave nature of the top also leans toward retaining something, like a person's neck, let their head hang over and whack or maybe a series of these in a row that you maybe would lay a rail on or a telephone pole or spar, so it won't roll while it is being worked on?.

This is at the house I inherited from my aunt and uncle. They had a masonry company about 60 years ago...if that makes a difference. I have no idea wtf to do with it, let alone dispose of it.

If it was made of cement...a few well-placed hits with a sledge-hammer would take care of it.