This adjective describes what cannot be spoken or adequately expressed in language. In Romans 8:26, the Spirit intercedes with groans too deep for words when believers don't know how to pray. The term emphasizes communication beyond verbal capacity. Classical Greek used it for unspeakable things, while the multilingual evidence (Spanish 'indecibles', French 'indicible', German 'unaussprechlich') confirms this sense of transcending linguistic expression, pointing to prayer's ineffable dimensions.