A dapper old gent spoke with me in the lagoon today; he mentioned in his native Germany that "they would say of a man like you, that he drinks from the..." Expressive pantomime followed, and then "from the gutters that hang down from the roof."
I allowed that it was quite a mental image; he thoughtfully added: "You don't want to drink that water." He was quite helpful in pointing out the remark was vaguely derogatory, but it would come as a surprise to me as to how.
The German saying I can find that most approximates the matter varies in its phrasing, but one version is:
"Wenn Dummheit lang machen würde, könntest Du aus der Dachrinne trinken!"
"If stupidity were height, you could drink out of the gutters."