A "dirty smile" bore what?

Although I'm not exactly sure what this question means by "bore," chapter five includes this passage:

"A sentence with variations went up and down the line. "Well, we 've helt 'em back. We 've helt 'em back; derned if we haven't." The men said it blissfully, leering at each other with dirty smiles."

Perhaps the answer to this question is something like: the dirty smiles bore a joyful recognition of success even in the midst of such terrible circumstances.