You know, Shakespeare might have used the word (I can't be sure, I don't that much about linguistics), but in that case Han could have repelled the charge instantly. 'cause the original meaning was not just unkempt, but having a scalp disease.
One of the commonest changes in words is for a specific word to become less precise and closer to "good" or (as in this case) "bad." In this case, it no doubt helped that the condition that it described became less common what with plumbing and stuff. Like "lousy." No one's spontaneous reaction to that is going to be "teeming with lice." Though others have lost their fire: "mangy" or "scurvy."
Of course that always introduces complications when you try to write about an earlier era and all the accurate terms have been pre-empted by later usage. . . though those two, you can get around with sanitary magic. 0:)