1. To befuddle, to confuse, to rattle, to mix up, to discombobulate: "The problem is that if anyone tries to use logical explanations for some political positions, it just addles the people and makes their understanding more confusing."

"While they were walking along the dark street late at night, their brains were addled by fear of what might happen to them."

2. To rot, to decay, to putrefy: "The eggs were addling because they were not refrigerated and were on the shelf too long."
3. Etymology: from a Middle English word from the Old English adela, meaning "liquid filth". It’s related to the German adel, meaning "urine, mire" or "puddle".