Earlier quoted context omitted.
Sorry, it was supposed to be rhetorical. It's a borrowing from Portuguese, so by the standards yumraj seems to be employing it shouldn't count as Hindi. Likewise, निश्चित certainly isn't Hindi in this narrow sense. It's a direct borrowing from Sanskrit, not a native, inherited word of the language. And Hindi (हिन्दी) itself isn't a Hindi word; it's Persian (likewise Hindu ).
Sorry, I didn't realise it was rhetorical. If we discount direct borrowings from Sanskrit, I guess there would be very little left. Something like Pali, essentially.
So 'fire' in Hindi(/Urdu) is āg (आग), but 'fire' in Sanskrit is agni (अग्नि), which has also been borrowed into Hindi (as the name of the god of fire; or fire in ritual contexts). Now, āg descends from agni, but represents the natural linguistic development into Hindi.