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 being a soft variant of the Persian "Kuh" ('mountain'). Allan states that, to Arab geographers, Hindu Kush was the frontier boundary where Hindustan started. Another theory suggests the "Hindu Kush" is derived from word 'Hindu' coming from "sindhu" meaning 'river' in Sanskrit, while "Kush" is sanskrit word related to Persian "Kuh" meaning ('mountain').Another possibility is that the name may be from the ancient Avestan language, with the meaning 'water mountain'.