A penny is a customary addition to knife gifts as it is considered bad luck to give a gift that can sever. The blade symbolizes the severing of a friendship or relationship, and since pennies are considered good luck, the superstitious belief is that the penny protects the relationship. A common superstition surrounding knives is if they are given as a wedding gift, the blade severs the marriage vows.
Another tradition that is similar to giving a good luck penny is the receiver "buying" the gift of knives from the giver for a penny. It is believed that this exchange breaks the curse and protects the relationship.
Pennies have been associated with good luck for thousands of years. In ancient times, while there were no pennies in existence, people believed that any source of metal found on the ground was a gift from the heavens that was sent down as a token of protection from evil forces. The superstition has evolved over time, and the phrase "find a penny and pick it up, all day long you'll have good luck" is derived from that single early belief. Finding a penny on the ground is also considered good luck because money is a symbol of power.