1. in a way that cannot be penetrated, entered, or passed through
2. in a way that is impossible to understand or comprehend

Examples

• The jungle was impenetrably thick with vines and vegetation.
• She stared at him with an impenetrably blank expression.
• The ancient text was written in an impenetrably complex code.
• The fog rolled in impenetrably, obscuring the entire coastline.
• His motives remained impenetrably hidden from everyone around him.
• The fortress walls were impenetrably fortified against attack.