The Business Standard tells us all about it:

Things that have fermented make some of the most appetising flavours on the modern table. Wine, beer, vinegar, bread, cheese, sauerkraut, achar, dahi, dosas and idlis all come from fermentation. Nice things (mostly) go into these foods. Would you, however, be quite as interested in garum, a fish sauce that the ancient Romans adored, and which was made of fermented fish entrails?

Perhaps not. But similar fish sauces are still around, especially in Southeast Asian cuisines. Vietnam’s ubiquitous nuoc mam is made much like garum was, and then diluted or flavoured with a variety of ingredients to go with many dishes.

The India-inspired English Worcestershire sauce is another. Worcestershire was invented quite by mistake in the 1830s, after a barrelful of an attempted anchovy sauce turned out too pungent and was left in a basement and forgotten. When it was finally opened years later, the liquid was discovered to be quite tasty, and Messrs Lea & Perrins (whose basement it was) marketed it very successfully.

This is how garum was made: fresh fish parts, the blood and innards of mackerel, say, were layered alternately in large containers with generous amounts of salt. Over about a month of standing in the sun, the enzymes within the fish broke them down into a thin liquid — the garum — and a paste that settled at the bottom known as allec. (Pliny the Elder writes that allec heals burns; it was also eaten as a savoury spread.) The process was so unbearably smelly that laws forbade Romans to make garum at home; thus one of Rome’s few suburban factory industries came into being.

The sauce itself was not strong-smelling, and seems to have been used very widely as a condiment, in place of salt. In fact, the Romans took it from the Greeks, whose garos was designed to avoid wasting all the assorted little fish at the bottom of the net which couldn’t be eaten as separate dishes.

As imperials, the Romans added snob value to the food, by discriminating between garums made from different raw ingredients, such as single species of fish, more blood, or more intestines (which added to the pungency). Garum is thus one of the earliest manufactured, processed foods with a global market. Ancient ketchup, in other words.

Fermented fish, anyone. Or garum, as the Romans liked to call it.

... never heard about the Greek origins before ...