peanut gallery

Short definition from Wiktionary

Noun

1. 1
   In the nineteenth century, the cheap seats at the back of a theatre or in the upper balcony.
2. 2
   The upper balcony to which black patrons were restricted in racially segregated venues such as theatres.
3. 3
   Any source of heckling, unwelcome commentary or criticism, especially from a know-it-all or of an inexpert nature. May also now refer to general audience response: "Let's hear it from the peanut gallery."