It’s ‘ketchup’, even though the English sailors who brought it back from Singapore in the 1600’s didn’t have the slightest idea of how the word should be spelt.
The original ketchup was the Chinese ke-tsiap, a pickled fish sauce. The Malays got hold of it and used the name kechup, only instead of using fish in the sauce, they used mushrooms.
When Americans got the recipe, they added tomatoes. Much later, Mr. Henry Heinz would be the first to make it a major product, and in 1876 introduced Heinz Tomato Ketchup. Since the Chinese, Malays, English and American concoctions all start with the “ke” sound, most word purists would tell you it is definitely ‘ketchup’, but never ‘catsup’.
In addition, several Journalists and bloggers dug around to find the history and reported that:
According to a Heinz spokesperson, Henry John Heinz first brought his product to market as “Heinz Tomato Catsup,” but changed the spelling early on to distinguish it from competitors. Del Monte did not switch spellings until 1988, after it became clear that ketchup was the spelling of choice for American consumers. Hunt’s switched the name of their product from catsup to ketchup significantly earlier.