Definition of "Fritz"

Someone sent up a shout and word came that oil had begun to show on the water—that meant one less submarine afloat! Again the roar of guns—again the shout! Two Fritzes sunk. . !

1918, Elaine Sterne, Over the Seas for Uncle Sam‎

[…] so Fritz could sleep easily in his trench, so far as mines were concerned; and Fritz, confident in his Herr Professors, altogether neglected any attempt to counter-mine.

1923, Frank Fox, “The Man-Made Earthquake”, in Charles Francis Horne, The Great Events of the Great War‎, volume 5

'Fritz is pounding away at the crossing,' he said. 'We'd do better to wait till things quieten down.'

1952, Vasily Grossman, translated by Robert Chandler and Yury Bit-Yunan, Stalingrad, page 227