According to M. F. K. Fisher in The Art of Eating, garum is an ancient Roman condiment made from rotting fish guts, similar to the Vietnamese nuocman. The Romans used garum in combination with spices. Fisher quotes the recipe as follows:
Place in a vessel all the insides of fish, both large fish and small. Salt them well. Expose them to the air until they are completely putrid. In a short time a liquid is produced. Drain this off.
Enjoy!