Closed book synonyms | phrases

Phrases with similar meaning for Closed book

• brain-teaser

  puzzle
• tough nut to crack

  brain-teaser, mystery
• brain-twister

  brain-teaser
• knotty question

  brain-teaser
• mind-boggler

  brain-teaser
• tough proposition

  brain-teaser
• head-scratcher
• magic number
• skeleton in cupboard
• privileged information
• confidential information
• unsolved problem
• question mark