extremely evil or cruel; expressive of cruelty or befitting hell
On stormy nights, when the wind shook the four corners of the house and the surf roared along the cove and up the cliffs, I would see him in a thousand forms, and with a thousand diabolical expressions.
a deliberate discourteous act (usually as an expression of anger or disapproval)
If ever he mentioned it, the captain blew through his nose so loudly that you might say he roared, and stared my poor father out of the room. I have seen him wringing his hands after such a rebuff, and I am sure the annoyance and the terror he lived in must have greatly hastened his early and unhappy death.
"Were you addressing me, sir?" says the doctor; and when the ruffian had told him, with another oath, that this was so, "I have only one thing to say to you, sir," replies the doctor, "that if you keep on drinking rum, the world will soon be quit of a very dirty scoundrel!"
I remember his breath hanging like smoke in his wake as he strode off, and the last sound I heard of him as he turned the big rock was a loud snort of indignation, as though his mind was still running upon Dr. Livesey.
Another definition of "feeble" is "pathetically lacking in force or effectiveness"--this also fits the example sentence because the captain, who is weak from a recent fight and stroke, is feebly cursing a doctor who is neither present nor afraid of him.
But he broke in cursing the doctor, in a feeble voice but heartily.
a sudden loss of consciousness resulting when the rupture or occlusion of a blood vessel leads to oxygen lack in the brain
Although the captain was already weakened by too much rum and a previous stroke, the "thundering apoplexy" that causes his death was triggered by his knowledge that he had been given the black spot and would be killed in six hours; so in a way, the captain died from a fear of death.
The captain had been struck dead by thundering apoplexy.
The neighbourhood, to our ears, seemed haunted by approaching footsteps; and what between the dead body of the captain on the parlour floor and the thought of that detestable blind beggar hovering near at hand and ready to return, there were moments when, as the saying goes, I jumped in my skin for terror.
Another definition of "formidable" is "extremely impressive in strength or excellence"--this does not fit the example sentence, because the adjective is referring to a blind beggar who is inspiring fear through his curses and commands. However, the narrator Jim and the author Stevenson could've intended some irony, especially since both know what happens to the beggar later in the chapter.
Four or five of them obeyed at once, two remaining on the road with the formidable beggar.
This appeal seemed to produce some effect, for two of the fellows began to look here and there among the lumber, but half-heartedly, I thought, and with half an eye to their own danger all the time, while the rest stood irresolute on the road.
Him they had deserted, whether in sheer panic or out of revenge for his ill words and blows I know not; but there he remained behind, tapping up and down the road in a frenzy, and groping and calling for his comrades.
As for my mother, when we had carried her up to the hamlet, a little cold water and salts and that soon brought her back again, and she was none the worse for her terror, though she still continued to deplore the balance of the money.
"Wretched" is an odd adjective to describe a medical practice. But it is coming from a squire who has traveled, and he is talking to a small-town doctor in the 18th century who must take on additional duties as a magistrate and whose income is not so great that a treasure hunt would not be tempting.
"Livesey," said the squire, "you will give up this wretched practice at once.