Catsup is just another way of writing ketchup (to make it sound more Anglo-American), which was originally spelled catchup. The origin for this is not known for sure, but there are several interesting theories. We know it was borrowed in 1690 from trade routes, and there is a Malay word, kichap, to describe a very similar condiment, but even that is likely borrowed from sea trade as well. There are cognates all around the South China Sea, indicating that the word might derive from that region. There's the Indonesian word ket-jap, meaning "soy sauce", there's the Malay word kicap, with the same meaning, and there's the Min Nan dialect of Chinese word koechiap, meaning "fish brine". The predominant theory is that all others derive from this latter one; if so, you can also see the evolution of the liquid from sour to sweet and tangy, and from fishy to tomatoey. I thought that was cool.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.