antagonist

 
Pronunciation: /anˈtag(ə)nɪst/

noun

  • 1a person who actively opposes or is hostile to someone or something; an adversary: the woman was forcing her antagonist’s face into the mud
  • 2 Biochemistry a substance which interferes with or inhibits the physiological action of another: LSD is a serotonin antagonistCompare with agonist.
  • 3 Anatomy a muscle whose action counteracts that of another specified muscle. Compare with agonist.

Origin:

late 16th century: from French antagoniste or late Latin antagonista, from Greek antagōnistēs, from antagōnizesthai 'struggle against' (see antagonize)