Originally, ketchup was a sauce made from the juice of mushrooms, tomatoes, walnuts, etc., used as a condiment with meat or fish.
The word ketchup appears to be from Chinese (Amoy dialect) kōetsiap, brine of pickled fish or shellfish, from kōe, seafood, and tsiap, sauce.
It has been claimed that the Malay kēchap (ketjap in Dutch spelling) is the original source of the word, but this Malay word is itself probably from Chinese.
The original Chinese word was anglicised as catchup and catsup - the latter being still in use in the USA.
For similar transformations of words, also read:
It is in the anglicised form catchup that the word first appeared in the English language in 1690. It was defined as “a high East-India sauce” in A new Dictionary of the Terms Ancient and Modern of the Canting Crew, by “B. E. Gent.”
The form ketchup is first recorded in 1711, and the form catsup in 1730, in a poem by Jonathan Swift, titled A Panegyric on the Reverend Dean Swift. In Answer to a Libel on Dr. Delany, and a Certain Great Lord (Dean Smith: another name by which Jonathan Swift was known.):
But when at last usurping Jove
Old Saturn from his empire drove,
Then gluttony, with greasy paws
Her napkin pinn’d up to her jaws,
With watery chops, and wagging chin,
Brac’d like a drum her oily skin;
Wedg’d in a spacious elbow-chair,
And on her plate a treble share,
As if she ne’er could have enough,
Taught harmless man to cram and stuff.
She sent her priests in wooden shoes
From haughty Gaul to make ragooes;
Instead of wholesome bread and cheese,
To dress their soups and fricassees;
And, for our home-bred British cheer,
Botargo, catsup, and caveer.
(Botargo is a relish consisting of the roe of mullet or tunny, salted and pressed into rolls. Caveer is caviar.)
In The Art of Cookery, made plain and easy (1751), Hannah Glasse gave two recipes for catchup. One, in the chapter For Captains of Ships, is titled To make catchup to keep twenty years. The other is as follows:
To make catchup
Take the large flaps of mushrooms, pick nothing but the straws and dirt from it, then lay them in a broad earthen pan, strew a good deal of salt over them, let them lie till next morning, then with your hand break them, put them into a stewpan, let them boil a minute or two, then strain them through a coarse cloth, and wring it hard. Take out all the juice, let it stand to settle, then pour it off clear, run it through a thick flannel bag (some filter it through brown paper, but that is a very tedious way), then boil it; to a quart of the liquor put a quarter of an ounce of whole ginger, and half a quarter of an ounce of whole pepper. Boil it briskly a quarter of an hour, then strain it, and when it is cold, put it into pint bottles. In each bottle put four of five blades of mace, and six cloves, cork it tight, and it will keep two years. This gives the best flavour of the mushrooms to any sauce. If you put to a pint of this catchup a pint of mum, it will taste like foreign catchup.