S. R. Faruqi:

sau sar kaa honaa = to be firmly resolved; to expend great energy and force in some task

The idiom sau sar kaa honaa is an Urdu one, it's not present in Persian. It's surprising that [the dictionary] nuur ul-lu;Gaat doesn't have it; and in farhang-e a;sar too there's no sign that it's missing. Platts, Duncan Forbes, and the aa.sifiyah have this idiom, but in the latter the meaning is not given with full accuracy. In the dictionary of fariid a;hmad barkaatii it appears, but the meaning is given incorrectly.

Mir has used this idiom a number of times; for example, in the first divan [{334,3}]:

us dard-e sar kaa la;Tkaa sar se lagaa hai mere
sau sar kaa hove .sandal mai;N miir maantaa huu;N

[that headache[-causing magic] wand has settled on my head
sandalwood might be 'of a hundred heads', Mir, I agree]

In the fifth divan [{1684,3}]:

jo sau sar ke ho aa))o maanuu;N nah mai;N
((aba;s khaate ho tum qasam par qasam

[if you would come 'with a hundred heads', I wouldn't agree
vainly you take/'eat' vow upon vow]

In the present verse, Mir has brought in his special style of operation: he has used the idiom in its dictionary meaning and created the form of a 'reversed' metaphor. That is, 'Mir' had only one head; the beloved cut it off and ended Mir's existence. But if Mir had had a hundred heads (that is, if one were cut off then ninety-nine would have remained, if two were cut off then ninety-eight would have remained, and so on), even then the beloved would have paid no attention to Mir and would not have spoken to him.

With regard to the idiom, the meaning becomes that Mir didn't achieve his ambition, or that with force and tumult and stubbornness he wasn't able to bring it to fulfillment; thus he couldn't bear the beloved's coolness or her inattentiveness, and ran away. But even if Mir had achieved his ambition, and had used force and tumult in his attachment, the beloved still would not have talked with him. She would have kept eating paan and biting her lips in anger.

In both cases, there's cheerfulness of disposition, and a slightly sarcastic tone. To suppose that the lover has a hundred heads, and to show the beloved chewing paan, is the perfection of a humorous style. It's a fine verse.

The theme of eating paan, and speaking while chewing it, Muhammad Aman Nisar has also versified. But his verse is very superficial:

itraa))o bahut nah paan khaa ke
baate;N nah karo chabaa chabaa ke

[don't boast a lot after eating paan
don't 'swallow your words']

This aspect of the theme, Mir himself has versified in a much better manner, in the fifth divan [{1781,9}]:

yih rang rahe dekhe;N taa chand kih vuh ghar se
khaataa hu))aa paan aa kar baato;N ko chabaa jaave

[this 'color'/style-- let's see how long it would last, if/when from the house she,
eating pan, would come and 'swallow her words']

With regard to paan, the double meanings of rang and chabaanaa are very fine. In Muhammad Aman Nisar's verse, there's only one instance of wordplay; in Mir's verse there are several instances. In Muhammad Aman Nisar's tone is bitterness and vexation. In Mir's tone, as usual, there's complexity-- there's melancholy, good-humoredness, a touch of irritation, and also acceptance of the situation.

FWP:

SETS == HUMOR; IDIOMS
MOTIFS
NAMES
TERMS == IDIOMS; METAPHOR

The first line is clear, since SRF explains and illustrates the idiom so clearly. But what about the second line? Here are some possibilities:

=The beloved might deliberately bite her lip, as a reflection of her anger at the persistence of the madly importunate ('hundred-headed') lover.

=The beloved might bite her lip in the sense of 'bite her tongue', to keep herself from expressing her vexation in words that she might later regret.

=The beloved might accidentally bite her lip, since she's chewing paan while being annoyed and distracted by the importunate lover

=The beloved might 'bite her lip' in the sense of the idioms chabaa chabaa ke baat karnaa or baat chabaa jaanaa (see the definitions above)-- since she refuses to speak, instead of 'chewing' the baat she would be chewing her lip.

It's hard to pin the possibilities down entirely, but this seems to be the general range. And they're all, in their own ways, rather funny.