Pewter and Brass Cartridge Flask
Truthfully, a flask isn't about discretion. If you really need to disguise your drinking, fill up a can of Pepsi with rum (and don’t forget your fake ID at home). A flask is subtle, but it's an assertion nonetheless that there's nothing salacious or vulgar about a few sips of drink.
Take out this pewter-and-brass, cartridge-shaped flask and everyone knows what's in there, but it also says that you take alcohol seriously, as something meant to be enjoyed and passed around.