Again, but a few days ago, Michael took the trouble of appearing at the counting-house door, hatless, in his shirt- sleeves, — his coat and castor having been detained at the public-house in pledge;
castor is an old word for beaver, the animal. according the the OED it also meant a hat made from beaver fur, but began to mean something supposed to resemble beaver but likelier to be rabbit (which presumably was less fancy) Charlotte appears to be using this humorously or sarcastically considering the context
