Read the following poem: 'Dubkiyaan Sindhu mein gotakhor lagaata hai. Jaa-jaakar khaali haath lautkar aata hai. Milte na sahaj hi moti gehre paani mein. Badhta doona utsaah isi hairaani mein. Muthi uski khaali har baar nahin hoti. Koshish karne waalon ki haar nahin hoti.' Question: What does the word 'Sindhu' mean?