S. R. Faruqi:

Mir has already composed this theme, identically, in Persian:

;xurramii ma((luum shud laf:z-e zabaan-e diigar ast
ii;N lu;Gat jaa))e nah mii-yaaband dar farhang-e maa

'I learned that "happiness" is a word of some other language:
When I searched for this word in my dictionary, it did not appear.'

If we overlook the fact that in the structure of the Persian verse 'Hindustani-ness' [hinduustaaniyat] prevails, the poetic beauty itself is less than in the Urdu verse.

The first line of the Urdu verse is insha'iyah. Then, in it there's also the suggestion that there's certainly something called 'delight of the heart', because if there weren't, then people wouldn't mention it. There's an additional implication that we have never attained 'delight of the heart', but neither has anyone even been able to describe it, so that we were obliged to ask people about it. After inquiry, we learned that there's no such word in our language.

In the Persian verse, by saying that there's no word 'happiness' in our dictionary, he has limited the idea, because it's possible that it might not be in our book, but possibly it might be found in someone else's dictionary. In Urdu, by expressing the idea absolutely that this word has not at all come into the Hindi language, he has proved the non-presence both of 'delight of the heart' and of the idea of delight of the heart.

In the whole verse despair, sarcasm, bitterness, an outer simplicity but an inner wit-- all these things have been dissolved into each other. By contrast, in the Persian verse there's a kind of artifice-- ma((luum hu))aa is a bookish phrase, and not free of formality. The Urdu verse is extremely casual.

And in a literal word-for-word way, suruur-e qalb is in any case, with regard to dictionary origins, not a 'Hindi' (that is, Hindustani) word. Both words are Arabic and the i.zaafat between them is Persian.

FWP:

SETS
MOTIFS
NAMES == HINDI
TERMS == IMPLICATION

Not only in Mir's day, but as late as Ghalib's day, 'Hindi', like 'Rekhtah', could be unselfconsciously used to refer to what is now called Urdu. (Ghalibian examples of both names can be found here.)

As SRF notes, there might be a small touch of faux-naïf humility too: 'What's all this fancy Persian and Arabic suruur-e qalb ? I'm a simple local boy, I only know our indigenous words.'

Compare Ghalib's example of a meaningless word:

G{9,1}.