1. Without touching the ground or without any patterns on the bottom of shoes, boots, or tires: It was obvious that Ned needed new shoes because they were completely treadless from years of use.
2. The parts of wheels or tires that make contact with roads or rails and which are without patterns of ridges, or grooves, and that are non-existent on tires: It is very dangerous to drive a car with treadless tires because it is much easier to slip or to slide out of control.