Mouth-watering means so tempting that your mouth prepares to eat. A mouth-watering smell, mouth-watering photographs.
The watering is real, since saliva starts at the smell or sight of food before anything is tasted. It is also used of anything desirable, so a mouth-watering offer has nothing to do with food.
1adjectivemouth-wateringSo tempting that your mouth prepares to eat.
“The smell from the bakery was mouth-watering three streets away.”
mouth-watering is a quality.
mouthwatering comes from mouth + water.
Etymology from Wiktionary, CC BY-SA
Practised in Smell & Taste