Suffix "ness"

Suffix "ness" - the state of, or condition of being something e.g., awkwardness = the state of being awkward.

So the Loch ness is the state of being a loch. Loch is the Scottish Gaelic and Irish word for a lake or a sea inlet. So Loch ness is the state of being a lake.

Or if we take Loch from Russian slang, it means a stupid person or something like that.
So in that case Loch ness would be the state of being a stupid person - which sounds closer to the truth.

Cheers!