The kingpin is the main pivot in the steering mechanism of a car or other vehicle. Originally this was literally a steel pin on which the moveable, steerable wheel was mounted to the suspension. In more modern designs, it may not physically exist as an actual pin, but the axis around which the steered wheels pivot is still referred to as the kingpin (or kingpin axis). It is usually made out of metal.
Henry Ford once commissioned a survey of all scrap yards in America to see what parts on his Model T were holding up best against wear. When the results came back, it was determined that the part which rarely or never broke was the kingpin. He reduced the quality of this to meet all other parts, and thus he conserved money for the company.