I answered this same question on ELU, and I think the answer is applicable here too (semi-paraphrased).
The word here is “damned”. The OED, in an entry that seems to have been written in 1894 and not been updated since then, noted this about the word:
Now usually printed ‘d——d’.
This was a pretty common way for vulgar words to be censored in the 1800s. You will also see, for example, “b——y” (bloody) and p—ck (prick) in books of this period.
Note that this is no longer common. First of all, damned isn’t considered too bad of a word anymore (not too polite but not so offensive that you need to protect people from seeing it), and words that are that offensive have asterisks used instead of dashes.