died in the wool 或者died-in-the-wool 是什么意思

Dyed in the wool refers to the color of the wool when it is still on the sheep. I.E. black yarn from a black sheep is died in the wool. It is not actually dies, it is part of it's nature and is unchangeable. Thus if someone was a dyed in the wool communist it would mean that it was deeply ingrained as it is part of his very character, unchangeable and perhaps even born into his beleifs.
直译过来就是“羊毛本色”，最终翻译成根深蒂固，与生俱来比较恰当。