According to Nigel Allan, the term Hindu Kush has been commonly seen to mean Hindu killer, but two alternate meanings are sparkling snows of India and mountains of India with Kush possibly a soft variant of Kuh which means mountain. Another theory suggests the word Hindu in Hindu Kush is derived from the same root as sindhu, meaning river, while Kush is a variant of the Persian word for mountain. Allan states that, to Arab geographers, Hindu Kush was the frontier boundary where Hindustan started. Another possibility is that the name may be from the ancient Avestan language, with the meaning water mountain.