From Hokkien Chinese 'ke-tsiap' (pickled fish brine) — the original had no tomatoes; that took two centuries of English tinkering.
A smooth, thick sauce made from tomatoes, vinegar, sugar, and spices, used as a condiment.
Most likely from Hokkien Chinese 'kê-tsiap' (鮭汁) or 'kôe-chiap,' meaning 'brine of pickled fish' -- a fermented fish sauce used in Southeast Asian cooking. British traders encountered the sauce in Malay-speaking ports of Southeast Asia, where it had been adopted as 'kecap' (a term still used in Malay and Indonesian for soy sauce). English colonists brought the word home and spent the next two centuries trying to recreate the sauce with local ingredients -- mushrooms, walnuts, anchovies
The original ketchup contained no tomatoes at all -- it was a thin, dark, fermented fish sauce more similar to Southeast Asian fish sauce or Worcestershire sauce. Tomato ketchup was not invented until 1812, and it did not become the dominant variety until the late 19th century. The thick red sauce we squeeze from bottles would be completely unrecognizable to the sailors who first brought the word to England.