An alternative theory as to the origin of the word ketchup (or catsup, if that be your fancy) supports
sensei's suggestion that the sauce was created by the
Chinese.
The Cantonese word pronounced ke-tsiap pre-dates the 1600s, and the time when tomatoes were first brought to China. Ke-tsiap means "fish sauce".
As for the Malay connection referred to by Andara, it is thought possible that the word was borrowed from the Chinese, along with the recipe for fish sauce.
By the eighteenth and nineteenth centuries, the word pronounced "ketchup" was used generically for any sauce containing vinegar. The word is first recorded in English in the year 1690 in the form catchup, in 1711 in the form ketchup, and in 1730 in the form catsup.
research source: American Heritage Dictionary, Third Edition 1992