The byproduct of smoking a lot in a vehicle. The smoke discolors the windows and effectively adds a layer of "tint" to the glass. This layer grows over time due to smokers not washing their vehicle. While effective in reducing harmful UV rays, the benefit is reduced by loss in resale value of the vehicle. Nicotint is also a sometimes unwanted deterrent to the opposite sex. Levels of second hand nicotine exposure are unknown at this time.
Johnny: Why is it as dark as a limo in here?
Leonard: Its due to the layers of nicotint Charles has added over the past 6 years.