armchair quarterback

noun2

1. •
   (US, informal) A person who watches sports on television, typically American football, who often verbally suggests plays or critiques the players and coaches.

   • My dad is such an armchair quarterback; he recognizes every bad decision the coach makes as soon as the play is over and he knows whether or not it worked.
2. •
   (informal, figurative) A person who criticizes something in which they are not involved and lack expertise.