gainsayer

noun

1. One who contradicts or denies what is alleged; an opposer.
2. A person who gainsays others; a disagreeable person.

   “First, The writer, positively asserts, "That Wood's halfpence were current among us for several months with the universal approbation of all people, without one single gainsayer".”

   “This urge to ensure the continued viability of UK science is balanced by warnings that such a system brings closer the day when computers will take over the running of the world and discard humanity. One such gainsayer is Bill Joy, inventor of the web language Java and chief scientist at computer giant Sun Microsystems.”