A flat-sided can made of metal used to store liquids, such as fuel or water.
War can depend on the little things, but you can't win one with them alone. In the second World War the Germans were notoriously careful with the little things, and the perfect example of this is the Jerry can.
During WW2 'Jerry' was a popular nickname used of Germans, and the Jerry can is in fact the German can. It was created to help German armor carry more fuel during quick engagements characteristic of blitzkrieg, when supply forces couldn't keep up.
The rectangular and sturdy design made the cans reliable and easy to stack in the sides of tanks or the backs of trucks. Three handles allowed one man to carry two to four cans, or two men to carry one can between them without the can tilting to the sides and hitting the carrier's legs.
The upper part of the can was designed so that the can could not be filled to the brim - although it was possible by tilting it. The resulting pocket of air was connected by a tube to the spout and made the liquid pour out smoothly. The pocket also kept the can afloat in water even when full.
The cork and the lever to open it were also of clever design: the rubber seal would not wear in use, and no tools were needed to make the can air-tight unlike with its American cousin. The cork could also be locked in when opened so that the holder could use both hands to pour out the contents.
The design was supposedly a great secret and German troops were ordered if necessary to destroy any Jerry cans before letting them fall to Allied hands.
During the desert battles of North Africa from 1940 to 1943 the can received a second role: it carried another precious liquid, fresh water. The Allied water canisters were inferior and whenever an intact can was found Allied soldiers would scavenge it. By this time the thingamajig got its name which has lasted all the way to the 21st century, and the Allies started to produce their own version.
The brilliant documentary film series on WW2, The World at War, episode 9, "Desert - North Africa 1940-1943"