In
greek Legend Achilles was a famous and brave warrior who fought at The Battle of
Troy, He was ultimately killed by an
arrow to the heel. When Achilles was young his goddess mother dipped him in the River Styx which made him almost immortal, he was held the heel and hence that part of his body didn'
t touch the river. Hence his heel was his weakest part and ultimately caused his downfall. This is an idiom used to describe a hidden weakness or fault in somebody which may be used to harm them.