S. R. Faruqi:

amal = hope

Auj bin Auq [((auj bin ((auq] (this is the correct name, but 'Auj bin Anaq' is famous, perhaps because in Arabic ((auq means 'neck') was, in Hazrat Musa's time, an exceedingly tall person. They say that when he would stand in the ocean, the water used to come up to his knees, and when he seized hold of fish from the ocean and lifted them up in his hands, they were raised so high that the fish were roasted by the heat of the sun, and they were what he lived on. Auj bin Auq was an infidel, and he met his death at the hands of Hazrat Musa.

After this explanation, it's not hard to see how appropriate and fitting it is to call unending hopes (that is, a person's longings) 'the leg of Auj bin Anaq'. An additional 'meaning-creation' is that Auj was an infidel, and Hazrat Musa killed him by striking his staff on Auj's ankles. That is, to permit hopes to grow beyond limits is not good; it's good only to strike them down and finish them off. But to achieve this a prophetic miracle, or at least prophetic courage, is required.

Shad Azimabadi had made good use of the image of hopes growing beyond limits as a 'magical serpent':

umiide;N jab ba;Rhe;N ;had se :tilismii saa;Np hai;N zaahid
jo to;Re yih :tilism ai dost ganjiinah usii kaa hai

[when hopes would grow beyond limits, they are a magical serpent, Ascetic
the one who would break this enchantment, oh friend-- the treasury is his alone]

But he wasn't able to sustain it, and the verse fell prey to the admixture of unnecessary words and extremely extended moral lessons. Everything was spelled out, and no subtlety remained in the verse.

In Mir's verse, by means of allusion [talmii;h] the image has been created ('Auj bin Anaq's leg')-- there's a contemptuous attitude toward the hopes, and 'meaning-creation' in addition to this. Then, there's the pleasure that in the tone, along with a strange kind of carelessness, is self-criticism as well.

FWP:

SETS == IDIOMS; MAGAR
MOTIFS
NAMES == AUJ BIN ANAQ
TERMS == 'MEANING-CREATION'

There's also a wonderfully ambidextrous use of magar . If we take it to mean 'but', then hope keeps moving on, 'but' it's as surely doomed as the leg of Auj bin Anaq. If we take it to mean 'perhaps', then we're speculating about its nature-- does it grow so implausibly gigantic because it's really (like) the leg of Auj bin Anaq?

Note for grammar fans: In modern usage, chalaa jaanaa more often than not means 'to go away, to move along elsewhere'. That sense is possible, but here the secondary meaning of something like 'to continue, to persist' seems more what's intended (see the definition above). Maybe usage might have shifted a bit, since Mir's day.