Hub caps is bro code for a woman's nipples or areolas. The circular nature of the areola along with the central nipple,or "hub",combined with the surrounding nipple bumps (montgomery tubercles or "auxillary nipples") tend to resemble
the wheel or hubcap on a truck or car.This
illusion is further enhanced if the nipple is pierced with a barbell making it look like a cotter pin in the "hub" place ther to prevent the nipples from flying off during wild,rough sex or exotic dancing. Along this line,
puffy nipples could be referred to as "baby moons".