When I was a lot younger, in the UK one served up tomato sauce with one’s fish and chips. On the other side of the Atlantic something called “ketchup” (occasionally for some mysterious reason spelled: “catsup”) was applied to hotdogs. Now, in the UK, one is also liable to hear the word ketchup used rather than tomato sauce.
I don’t like tomato sauce so cannot be sure whether it is the same as ketchup or something different, but it is clear why tomato sauce is so called. Can anyone please enlighten me as to where the word ketchup came from, and the even more mysterious catsup?

dxb