Why Do We Say That?

ACHILLES HEEL
In Greek mythology Thetis dipped her son in the mythical river Styx. Anyone who was immersed in the river became invulnerable. However, Thetis held Achilles by his heel. Since her hand covered this part of his body the water did not touch it and so it remained vulnerable. Achilles was eventually killed when an arrow hit his heel.

RULE OF THUMB
This comes from the days when brewers estimated the temperature of a brew by dipping their thumb in it.

RED HERRING
Poachers and other unsavoury characters would drag a herring across the ground where they had just walked to throw dogs off their scent. (Herrings were made red by the process of curing).

SHAMBLES
Originally a shamble was a bench. Butchers used to set up benches to sell meat from. In time the street where meat was sold often became known as the Shambles. (This street name survives in many towns today). However, because butchers used to throw offal into the street shambles came to mean a mess or something very untidy or disorganised.

SET YOUR TEETH ON EDGE
This is from Jeremiah 31:30 'Every man that eateth the sour grape, his teeth shall be set on edge'.