A hip flask is a traditional way to carry around alcohol - usually a spirit (eg whisky, brandy etc) in a smart way.
Most hip flasks are quite small, but enough for one person to have a few drinks. Often made of silver or chromed metal, with a leather cover for decoration. Some come with small cups to make sharing between friends easier.
Really good hip flasks have a glass lining. This prevents any "metallic flavour" leaching from the metal body into the drink.
Some would say that anybody who owns (or carries around) a hip flask is a drunkard. I'd say check the contents. If it's a generic low quality drink, perhaps. But if it's a single malt whisky, or a decent Russian vodka, or a French cognac, maybe they are just someone who appreciates a classy drink!
wertperch points out that quite a lot of people collect hip flasks. /msg me if you know what the term used to refer to someone like this is!
StrawberryFrog says hip flasks are a good way of taking some fine liquor to where it's needed for a celebratory dram. By coincidence, I aim to be on top of Ben Nevis on this coming Sunday, with a hip flask full of single malt to toast the summit. There seems to be a tradition of this - the Malt whisky club gift shop says "This hip flask looks great on display, in a sporran or even at the top of a Munro!"