1. Die Katze im Sack kaufen - buy the cat in the sack - what Sigurd said
  2. Die Katze aus dem Sack lassen - let the cat out of the sack - what Sigurd said
  3. Ist die Katze aus dem Haus, tanzen die Mäuse auf den Tischen - If the cat left the house, the mice are dancing on the tables - if somebody "in chief" left, others will do what they like
  4. Bei Nacht sind alle Katzen grau - By night all cats are grey - in certain circumstances, one can't make a proper distinction
  5. Katzentisch - cat table - usually in a restaurant a sub par table, mostly near the toilet
  6. Katzenwäsche - cat wash - instead of having a proper shower, one just washes axles and face
  7. Kätzchen - kitten - seldomly an intimate name for an attractive woman, positive or ironically
  8. Muschi - pu**y - name for the female sex organ, pet name for a cat (very seldom) or for a woman (extremely seldom)
I think there's absolutely nothing negative within the German language regarding cats.