Hard puzzle

• Quite a puzzle
• Baffled
• It's not easy
• Difficult question
• A particularly difficult or baffling question or problem
• Tough one
• It's hard to answer

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Stumper \Stump"er\, n. One who stumps. A boastful person. [Slang] A puzzling or incredible story. [Slang, U.S.]

Wiktionary Word definitions in Wiktionary
n. 1 One who stumps, or speaks, or orates, as a politician. 2 A difficult puzzle or problem. 3 (context archaic slang English) A boastful person.