唐 詩 Tang Shi – Poèmes de l'Époque des Tang

Wuding means "un-fixed" or "shifting" and probably refers to the fact the sands in the desert shift, causing rivers to change course.

Thinking only of their vow that they would crush the Tartars- -
On the desert, clad in sable and silk, five thousand of them fell....
But arisen from their crumbling bones on the banks of the river at the border,
Dreams of them enter, like men alive, into rooms where their loves lie sleeping.

The above translation was embellished with some "poetic license".

Literally, the words meant:

Pledged to sweep the Xiong-nu away without fear for their own safety;
Five thousand clad in sable and brocade perished in the dust of Hu;
Pity the bones littering the banks of the Wu Ding River,
they were the very people dreamt of in ladies' bedchambers.