I am reading The Last of the Mohicans. A group gets lost on their way to Fort Edward. Then they meet Hawkeye the scout, who accuses their guide of misleading them. Hawkeye believes the guide's heritage confirms this when he says:
"A Huron!" repeated the sturdy scout, once more shaking his head in open distrust; "they are a thievish race, nor do I care by whom they are adopted; you can never make anything of them but skulks and vagabonds. Since you trusted yourself to the care of one of that nation, I only wonder that you have not fallen in with more."
I cannot find a definition of skull to match this usage. A definition I've found in dictionaries that is closer to Hawkeye's intent than the common meaning of "bone of the head" is this one from MacMillan Dictionary:
INFORMAL a person's head, or their mind
Especially if used in this derogatory sense cited from CED:
derogatory often the head regarded as the mind or intelligence: to have a dense skull
Hawkeye is not shy with his criticisms of this tribe. In the above passage alone he accuses them of thievery, aimlessness, and treachery. It is possible that by using skulls he is insulting their intelligence too. But this argument seems weak to me, because the traps and ambushes Hawkeye fears the guide leading the group into require some sophistication to execute.
A supposition I think has more merit is that this skull is related to skulduggery, which MW defines as
underhanded or unscrupulous behavior; also: a devious device or trick
The skul in skulduggery ("apparently an alteration of Scottish sculdudrie 'adultery' (1713), sculduddery 'bawdry, obscenity' (1821), a euphemism of uncertain origin") appears to have a different etymology than the bone-related skull ("probably from Old Norse skalli 'a bald head, skull,'..."). But I cannot find any sources to confirm the connection of Hawkeye's use of skulls to this skul.
What does Hawkeye mean by calling the tribe "skulls"?
Edit: As Leon Conrad points out below, I misquoted the Google book I cited. The correct word is skulks. (I have changed it above.) I have e-mailed the source I read from about the typo.