In the past people believed that bees flew in a straight line to their hive. Some added a loaf to a batch of a dozen to be above suspicion. It is said to come from the days when bakers were severely punished for baking underweight loaves. In Psalm 17:8 the writer asks God ‘keep me as the apple of your eye’. In Genesis Cain murdered his brother Abel. Like many phrases in the English language this one come from the Bible. Achilles was eventually killed when an arrow hit his heel. Since her hand covered this part of his body the water did not touch it and so it remained vulnerable. However Thetis held Achilles by his heel. Anyone who was immersed in the river became invulnerable. In Greek mythology Thetis dipped her son in the mythical River Styx. Unfortunately it is very often impossible to say for certain where an old expression comes from. I should point out that this list of old sayings gives their most likely origin. SOME OLD EXPRESSIONS AND WHERE THEY COME FROM