Jemandem reinen Wein einschenken. To serve someone pure wine.
This expression originates from the Middle Ages, when innkeepers added various ingredients to their wine (e.g. sulphur, acetic clay, water, etc.) and sold it as wine. In this way, the innkeeper could make even more out of the limited supply of wine. Only if he poured real wine, as he claimed, was he telling the truth.