An Etymological Dictionary of the German Language/Annotated/Dietrich
Dietrich, masculine, ‘false key’ (in Upper German Nachschlüssel), occurs late in Middle High German; the age of the word and of its meaning is attested by the loan-word Swedish dyrk (Danish dirk), which has the same signification, and is, like the Modern High German proper name Dierk, ‘Derry,’ a pet name from Dietrich, ‘Derrick.’ Similarly, instead of ‘Dietrich,’ Peterchen (Peterken), ‘Peterkin,’ and Klaus (Klöschen), ‘Nick,’ are used, probably because Peter, ‘Peter,’ like Dietrich, ‘Derrick,’ and Nikolaus, ‘Nicholas,’ are favourite Christian names, which might serve to veil (in thieves' slang?) the term ‘false key’ (compare Italian grimaldello) The word in Middle High German is miteslüȥȥel, Old High German aftersluȥȥil.