Literally: “donkey bridge” / meaning: mnemonic, learning technique
Nie ohne Seife Waschen (“never wash without soap”)—for many Germans, that’s the first Eselsbrücke they learn. It’s a perfect way to remember the 4 cardinal directions in clockwise order (Norden, Süden, Osten, Westen).
When you visit a school in Germany, there is even a chance your geography teacher will build you the Eselsbrücke for remembering the difference between stalagmites and stalactites by comparing the latter to saggy boobs. I guess this one works in English as well if you have a minimum of kinky imagination.
In any case, the origin of the word Eselsbrücke has nothing to do with donkeys being stupid, but rather a bit stubborn. They vehemently refuse to wade through even the shallowest of waters, so their owners have to build an improvised bridge to make them cross a creek instead of walking a long way around. In this sense, the word reflects the little extra effort it takes to learn a memory hook—which in the end pays off quicker than if you try to wrap your head around a complicated issue. Like washing your hands with soap.