A flask or a hip flask is a small container that is meant to hold alcohol. It can be made from pewter, silver, glass, or stainless steel.
In 1931, during Prohibition, the gangster Kid Tannen used a flask to hold his bootleg hooch. Marty McFly picked up this flask and gave it to Jennifer Parker in 1986 in exchange for a tire iron. In this timeline, Lorraine Baines McFly had a drinking problem, and would sneak a drink in a flask. Marty took the flask, and assured his mother that he wouldn't tell his father about it.
When Marty and Emmett Brown left 1931, they took off in the second DeLorean time machine towards 1986. The DeLorean was parked in the "Cars of the Future" exhibit at the Hill Valley Science Expo, in order to be hidden in plain sight. The judge of the contest tried to tell Doc Brown that he won the contest. However, when he missed him and saw the DeLorean fly away and break the time barrier, he took a sip of alcohol from a flask.
In the timeline where Jennifer Parker ran Douglas J. Needles into a delivery truck full of eggs in order to keep him from destroying the steam time car, Needles was arrested by two Hill Valley Police officers, Officer Foley and Officer Reese. The driver of the delivery truck had called the police, and told them of a "1920's car with a parachute". However, since the police found a flask in the delivery van, his testimony was not admissible in court.
