I saw this thread this morning: I fell on my keister yesterday. Having taken German in high school and college, I thought it was spelled wrong. In German, ie makes the ee sound, and ei makes the eye sound. (Ei, by the way, is the German word for egg. But I digress.)
I found out that Kiste, pronounced kee-steh, is a German word for a satchel/suitcase/chest, especially one secured with straps and a lock. It’s from the Latin ciste, from which we get the English word chest. Kiste, meaning buttocks, is recorded in Hans Ostwald’s Rinnsteinsprache (Gutter Speech ), a slang dictionary from 1906. The English word keister is attested to in 1881, as a name of a certain conman.
So while it’s not clear how keister, as in a locked satchel, became a slang word for the buttocks, it is plain to see how the English word keister, pronounced kee-ster came from the similarly-pronounced German word Kiste.