The tomato sauce, whose name is an archaic Hokkien Chinese word for fish brine: kê-tsiap, pickled-fish juice, carried across Southeast Asia and back.
Documented
The Hokkien record
James Murray traced it in the OED in 1889: Amoy-dialect kôe-chiap or kê-tsiap, brine of pickled fish, attested in Carstairs Douglas's 1873 Chinese-English Dictionary of the Vernacular of Amoy. English picked it up in Southeast Asia, first attested as catchup in 1699, with Charles Lockyer's 1711 trade account praising the best Ketchup from Tonquin. The Malay form kecap may itself come from the Chinese.
Assessment: Documented: the OED entry, Douglas's dictionary, Lockyer 1711. The syllable tsiap is the word for sauce.
Stories
The recipe drift
The documented mutation: English cooks, lacking the original, made ketchup from mushrooms, walnuts, oysters, and anchovies, Samuel Johnson's 1755 Dictionary treats mushroom ketchup as an imitation of the Asian original, and the first tomato ketchup recipe appears only in 1812. The fish disappeared from the sauce but stayed in the name.
Assessment: Documented: Johnson's Dictionary, the 1727-1742 Compleat Housewife recipes, the 1812 tomato turn.
Interesting claims
The round trip
The linguistic homecoming: the sweetened tomato form spread to Japan as kechappu, and during the colonial period Taiwanese Hokkien borrowed it back as khe-tsiap-puh, the word returning to its mother language wearing four hundred years of travel, unrecognizable to its own root.
Assessment: Interesting: a word that circumnavigated the planet and came home foreign. The archive's favorite itinerary.
Sources and further reading
Oxford English Dictionary, ketchup/catchup: from Amoy Chinese kôe-chiap or kê-tsiap, brine of pickled fish (Murray's 1889 entry)
Carstairs Douglas, Chinese-English Dictionary of the Vernacular of Amoy, 1873 (the Hokkien attestation)
Charles Lockyer, An Account of the Trade in India, 1711 (first English ketchup citation); B.E., New Dictionary of the Canting Crew, 1699 (catchup)
Samuel Johnson, A Dictionary of the English Language, 1755 (mushroom ketchup as imitation); first tomato ketchup recipe, 1812
Worth asking
How did a Chinese fish sauce become the most American condiment?