A tea flask is a container item that can store up to 5 cups of plain tea, as obtained from the tea stall in east Varrock. Tea flasks can be obtained by killing any of the hybrids in the Tower of Life. Like satchels, they do not stack in the bank even if they have matching contents. Players can drink the tea directly from the flask, the character saying, "Ahhh, tea is so refreshing." The tea also boosts Attack temporarily by 3 levels and restores 3 hitpoints just like when drinking from a cup. Players can use the tea flask on an empty cup, to fill the cup with tea instead of drinking from the flask.
Players cannot fill a flask with nettle tea or tea from a player-owned house.
For an exhaustive list of all known sources for this item, see here