My cat has a scabby bump on its back that oozes pus. What is it?

I'm glad to hear that Mr. Tibbs does not seem to be bothered by this bump. This lesion could be an abscessed wound, infected mass, infected cyst, etc. Anytime pus is present, this is indicative of white blood cells working to fight infection. I recommend taking Mr. Tibbs to his vet both for a course of antibiotics and to determine the cause of this lesion. He may need to have a needle aspirate performed to draw out a few cells with a needle and then examine them microscopically. Until you can get to the vet, you can try applying warm compresses to this area and be sure to keep an e-collar ( http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=20390 ) on Mr. Tibbs to prevent him from licking the area, because this will make matters worse. I hope that all goes well!