I did some googling for the origin of the word “heel”, and came up with this explanation:
“HEEL began its life in the U.S. underworld 1) meaning a sneak thief, perhaps a) from the idea that a heel was as low as one could get on the human anatomy, and this type of petty criminality was considered one of the lowest forms of criminal – close to the bottom of the barrel – among criminals themselves. b) because it describes an informer, which was considered to be the lowest of the …