The term "Low German" comes from the translation of the (modern German)
Plattdeutsch (literally "flat
German").
Plattdeutsch was spoken in northern
Germany, whose topographic composition is mostly
plains;
lowlands.
"Low German" is generally contrasted with High German, which was historically spoken in Germany's more altitudinous southern terrain, including the Alps.
Note that the terms "Low" and "High", as used here, do not refer to social class, only altitude.
Source: Word Mysteries and Histories. Jost, David A., PhD., Ed. 1986. Boston: Houghton Mifflin, p. 146.