This adjective means difficult to interpret, explain, or express—something challenging to make clear to others. Hebrews 5:11 uses it when the author says certain teachings about Melchizedek are 'hard to explain' because the readers have become dull of hearing. Classical Greek also used it for things hard to describe or interpret. The difficulty isn't in the subject itself but in communicating it to an unprepared or inattentive audience. The author's frustration points to the readers' spiritual immaturity, not the doctrine's inherent obscurity.