General terms:
* Inscrutable: This word implies a complete lack of understanding, suggesting something is deeply mysterious and impenetrable.
* Unfathomable: Similar to inscrutable, this word suggests something is too deep or complex to grasp.
* Enigmatic: This word suggests something is puzzling and mysterious, often hinting at a hidden meaning.
* Mysterious: This is a general term for something that is difficult to understand or explain.
* Perplexing: This word emphasizes the confusing and bewildering nature of something.
More specific terms:
* Abstruse: This word implies something is difficult to understand due to its complexity or obscurity.
* Arcane: This word suggests something is understood only by a select few, often due to its esoteric nature.
* Cryptic: This word implies something is intentionally hidden or unclear, often suggesting a hidden message or meaning.
* Obscure: This word suggests something is not well-known or easily understood, often due to its lack of clarity.
* Unintelligible: This word specifically means something that cannot be understood, often due to its lack of clarity or meaning.
The best word to use will depend on the specific context and the tone you want to convey.