antiknock

/AE0 N T IY0 N AA1 K/
noun
  1. 1

    Such a substance, usually tetraethyl lead (also MMT, MTBE and ethanol)

adjective
  1. 1

    Of an additive added to petroleum/gasoline to reduce the occurrence of engine knock.

Source: https://en.wiktionary.org/wiki/antiknock