The Legendary cocktail is a quest item used in The Fremennik Trials quest. It is obtained by giving a promissory note to Thora the Barkeep in Rellekka. The cocktail can then be given to Manni the Reveller in exchange for a champion's token. If one asks Manni what it tastes like after giving it to him, he'll say it was mostly like tomato juice.
The examine text, "Probably the greatest cocktail in the world", is a reference to the well-known (in the UK at least) slogan for the Carlsberg Group's popular lager. The tagline "Probably the best lager in the world" has been used by the company since 1975.