إِذَا وُضِعَتْ عَنِ الْأَبْطَالِ يَوْمًا رَأَيْتَ لَهَا جُلُودَ الْقَوْمِ جُونَا
When the coats of mail are taken off the warriors, you will see that the skin beneath them has become dark.
Wuḍiʿat is a passive verb: the armour has been taken off. Its implied referent is the coats of mail described immediately before. Jūnan here conveys a darkened appearance.
The mark left by prolonged armour-wearing becomes an image of sustained readiness. The address raʾayta brings the audience close to the scene as an imagined eyewitness.