Short answer: In German youth slang, 31er (Einunddreißiger) means a person who betrays others or acts as a snitch. It refers to someone who reports or informs on friends to protect themselves, and it derives from Paragraph 31 of the BtMG (Betäubungsmittelgesetz), which is linked to mitigating sentences when information helps authorities. The term is closely related to the English word “snitch” and is used pejoratively to label someone as a Verräter (traitor) or Nestbeschmutzer (one who fouls the nest) in casual speech.
If you’d like, I can tailor a short German-English mini-glossary entry for language learning or provide usage examples in context.
