Slang for “Grease Fitting” - Allegedly named after the patent holder ¹⁾ …. http://en.wikipedia.org/wiki/Grease_fitting

Zinc Dialkyl Dithiophosphate is the most commonly used anti-wear additive in motor oils. It is in a family of coordination compounds developed in the 1940s that feature zinc bound to the anion of dithiophosphoric acid²⁾ that work together to provide anti-wear protection and minimize lubricant breakdown and it also exhibits mild extreme-pressure protection. As temperatures rise and surfaces come closer together, ZDDP decomposes, and the resulting chemistry protects critical metal surfaces as it sets between metal to metal contact. Due to EPA regs, this is being phased out of motor oils. ³⁾