Liquamen was a Roman fish sauce.
Liquamen may originally have been a fish sauce different from the other Roman fish sauce, Garum. Some speculate that unlike Garum, Liquamen was originally just made from whatever fish and fish bits. The Romans were real snobs about food, and the fact that it wasn’t recorded in the 1st century AD may be that they didn’t think it was worth talking about, though that is pure speculation.
By the 400s, though, the word Garum had disappeared from usage, and been replaced by Liquamen. Apicius uses Liquamen in his recipes in the 300s. It may have just become a generic word for fish sauce.
Though there were many centres of Liquamen manufacture, such as Portugal and Spain, whose sauces were prized and sent all over the Empire, the Empire was a big place, and many people would have made their own versions with the fish they had to hand. Neither Garum nor Liquamen would have had a strong fishy taste at all, unlike today’s fish sauces.
In the 900s, the Byzantines were still making Liquamen. It disappeared, though, when Byzantium was conquered and occupied by the Muslims.
The thing to remember about Liquamen is that in fact it would not have tasted very fishy (see entry on Garum). Some suggest for 2 oz (50 ml) Liquamen substitute 1/2 tsp salt plus 2 oz (50 ml) white wine.