A Flask (sometimes called a hip flask) was a small metal container that was used to hold alcohol. The flask is usually held in a pocket.
When Cervantes Quinn had given up drinking, he often wished for a flask. (VAN - Declassified novella: The Stars Look Down)
George Pimintel drank from a flask in a holodeck recreation on the USS Enterprise (NCC-1701-E). (TNG graphic novel: Forgiveness)