A drink or cocktail. Stems from the concept of sharing the profits from a good harvest among the farmers involved, as in a glass of wine from a barrel or a shot of whiskey from a cask. Commonly used to refer to a drink poured from a larger stash (i.e. a bottle of whiskey or a batch of martinis).
Bob: "Yes, yes, Carolyn, I think equities are a terrible place to have your money these days. I could use another dividend. Bowmore, that will do!"
Carolyn: "I'm not going to have one myself but would be happy to get you one."