I haven't posted one of these in forever, so here you go :D
Why do we use the phrase "Achilles' heel" to refer to a man's vulnerable spot?
Because Achelles' vulnerable spot was his heel. According to Greek mythology, Thetis, the mother of Achilles dipped him into the River Styx in an attempt to render him invulnerable. Unfortunately, she held him by his heel as she did so. And thus his heel became his one vulnerable spot. Achilles died of a wound received from an arrow shot into his heel.