Samaa Zeitoun (asker) Oct 27, 2014:

I'd like to thank you, dear colleagues, for your answers. I was suspicious about this term bec. it does not rhyme with the rest of the poem, so I searched for the verses online and discovered that the source text I have is full of typos. Here's the correct text:
أما وقد خلع المرائي ثوبَه فليخلعنّ الغمدَ منه المِبترُ
وليلبسنّ الأرجوان غلالة تُطوى على هام الرجال وتُنشرُ
ولتعركنّ الظالمين سنابكٌ حتى يحجّبهم دمٌ لا عِثيَر
So, it turned out to be عثير which is "dust" and there is a لا before it. Now I have a problem with the meaning of المبتر here so I'll post it in a new question. Thank you again for your effort.