I don't believe it's in use in modern Hindi, but I think the Sanskrit word was perhaps तपति? This is just a shot in the dark though, as I don't know Sanskrit. (https://en.wiktionary.org/wiki/%E0%A4%A4%E0%A4%AA%E0%A4%A4%E0%A4%BF#Sanskrit).
I can say, however, that the root of the word is tep*, from PIE, which is where we get 'tepid' in English :)
EDIT: I asked my Indian gf and she said it's गुनगुना, which means lukewarm/warm, but can't be used for things like weather (only water/food, etc.).
When I was in New Delhi and asked at a shop to buy a bottle of water. The guy reached to give me refrigerated water. I said no. Then he handed me a different bottle and said Garam, and I understood it to mean room temperature. For the rest of my trip I asked for garam pani, and got what I wanted. So, is this another meaning of garam? It certainly was not hot water.
My response was also marked incorrect, but it seems the definite article (The milk...) is demanded. Google translate (which cannot always be trusted, admittedly) accepts our answers. Just as in English, hot and warm are relative terms.(Just tune into the weather reporter in July, and then in January in New England to experience a 60F degree cold day, and a 60F degree warm day respectively.)