The story
From Mennonite + Low German, because Mennonites were the ones to carry it out of Prussia.
Etymology from Wiktionary, CC BY-SA 4.0.
Mennonite Low German
Today it means: A variety or dialect of Low Prussian of East Low German with Dutch influence that developed in the 16th and 17th centuries in the Vistula delta area of Royal Prussia and which is now spoken by communities not only in Germany but also in North America (Canada, the United States, and Mexico), South America (Argentina, Brazil), Russia and many other places. (Not to be confused with the High German language Pennsylvania German; both are spoken in Pennsylvania.) Full definition →