I had some rather weak coffee this morning which reminded me of the German „Blümchenkaffee“ (lit. little flower coffee).
The term most likely comes from the fact that one could see the little flower at the bottom of the coffee cup because the coffee was so weak.
The flower design „Gestreute Blumen“ (strewn flowers) comes from the Meißen Porzellanmanufaktur in Saxony and was very popular during the Biedermeier in the early 1800s. At that time the expression „Blümchenkaffee“ was probably used to show the disconnect of the rather expensive porcelain and the cheapness of a host to not use enough ground coffee to make a decent coffee.
Photo: By makemake - Self-photographed, CC BY-SA 3.0, Wikimedia Commons 32846301
I personally know the term more in reference to coffee during and after the war when coffee beans were rare and were substituted with beets or barley, also acorns. This coffee was called „Ersatzkaffee“ or „Muckefuck“ (please pronounce German 😉).
Other words for weak coffee are „Plörre“, „Lorke“, or „Lurke“. You can use „Plörre“ also for other watery drinks, like weak or flat beer.