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".

A dimwit, an idiot, an imbecile, a moron: "Oh, no, I'm an addle-brain. I didn't mean to put salt in the coffee instead of sugar."

"There are too many addle-brains who are thinking about running for President of the U.S."

1. Dull-witted, stupid, and confused: "Someone who is obviously addle-brained is Democratic Congressman Anthony Weiner, of New York, who said he had been engaged in several inappropriate electronic relationships with six women over three years, and that he publicly lied about a photo of himself sent over Twitter to a college student in Seattle over a week ago."
2. Crazy, weak, mentally deficient in some way: "Is it possible that Dominique Strauss-Kahn, the former managing director of the International Monetary Fund, was addle-brained when the hotel-cleaning woman went into his room while he was still naked and he forced her to have oral sex with him?"