This adjective describes something intellectually difficult to grasp or understand. In 2 Peter 3:16, Peter acknowledges that some things in Paul's letters are 'hard to understand,' which the ignorant and unstable twist to their own destruction. Classical Greek used it for things difficult to comprehend. Peter's candid admission validates the intellectual challenge of some biblical teaching while warning against using difficulty as license for distortion. Difficulty requires diligent study, not revision to match our preferences.