A step beyond mouth-watering. Something is so desirable that it is literally mouth water.
A noun used to refer to something extremely good. Typically used to refer to non-food items or ideas.
Used in a similar way to "fire" i.e. 'that shit is fire.'
(1) 'Steph Curry's jump shot is mouth water"
(2) 'that ass is mouth water'
(3) 'The latest draft of the financial plan is mouth water'