Synonyms of closed book

closed book

noun

as in mystery

something hard to understand or explain

the couple's unorthodox marriage is definitely a closed book to those outside of the relationship

Synonyms & Similar Words

• mystery
• puzzle
• enigma
• secret
• riddle
• why
• conundrum
• mystification
• problem
• puzzlement
• challenge
• matter
• head-scratcher
• trouble
• poser
• perplexity
• case
• knot
• stumper
• brainteaser