all those who go through this post, i strongly recommend to have this dictionary as it includes the etymology of the idiom which plays an important role in the understanding and retention of the idiom and apply it in real time...
Like Achilles heel has a story behind it that there was a mother of a prince who never wanted to lost her son so she dipped her in some holy water to make him immortal which he became but the heel from which was holding that boy left dry so in a war an arrow penetrated into his heel and that wound lead him to die...so Achilles heel means a fatal weakness or a single flaw...