A pacifier was a device developed by the Minyans to reduce aggression.
While about the R1C, Orfe used a pacifier on Leela and Herrick. The Fourth Doctor commented that they had finally invented the device to which Jackson replied, too late and too few. The Doctor then queried as to how long the effect on Leela would last and, seeing as she was a primitive, Jackson told him it would be several hours before she returned to normal. (TV: Underworld)