The Tngufoss is a waterfall in the river Holtsá in the south of Iceland near Holt along ringroad 1.
The river Holtsá runs from the glacier Eyjafjallajökull where the Tngufoss falls from a cliff into a big ravine. It is appr. a 2 hour walk to the Tngufoss starting from the ringroad at Holtsá.
Unfortunately I can’t find the source that mentioned this waterfall anymore, but on the map I see a potential waterfall, so I assume it is really there.