A Scots/Scottish insult translating roughly as “Your father is a cosmetics salesman.” Avon was a cosmetics company that operates like a pyramid scheme, though it is completely possible to be an Avon lady (or an Avon man in this case) and make a profit the legitimate way without recruiting new members.
Scottish insult: "Your father is a representative of the Avon company"
The peak, the maximum, the be-all and end-all of humiliation. The ultimate insult of one's self, one's family and one's dignity. Consider all hope lost in your life if this is said to you. Use sparingly against enemies.