An Etymological Dictionary of the German Language/Dietrich
Dietrich, m., ‘false key’ (in UpGer. Nachschlüssel), occurs late in MidHG.; the age of the word and of its meaning is attested by the loan-word Swed. dyrk (Dan. dirk), which has the same signification, and is, like the ModHG. 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’ (comp. Ital. grimaldello) The word in MidHG. is miteslüȥȥel, OHG. aftersluȥȥil.