To confuse or muddle someone's thinking; to make something unclear or rotten (as an egg).
From Old English 'adel' (mud, filth) or possibly Scandinavian origin. Originally meant 'to become muddy or filthy,' then evolved to mean 'to confuse' metaphorically, like mud clouds water.
When an egg goes 'addled,' the inside becomes a soupy, confused mess—and that's exactly where the word comes from! Calling someone 'addled' means their brain has become like a rotten egg, which is pretty insulting when you think about it.
Complete word intelligence in one call. Free tier — 50 lookups/day.