a violently fast stream of water (or other liquid)
"Wuther" is a word used only in British English, and it means "to blow strongly with a roaring sound"--the wuthering and torrential weather matches Mary's miserable and angry mood, because it prevents her from working with Dickon in the secret garden. It also matches Colin's fretful crying, motivates Mary to find the crier, and foreshadows a later conflict between the two.
It was pouring down in torrents and the wind was "wuthering" round the corners and in the chimneys of the huge old house.
make psychologically or physically used (to something)
Most people, especially children, are not accustomed to the idea of dying. But the key phrase here is "as if" because Colin really doesn't want to die, but everyone keeps telling him he will, so the words have ceased ("have an end in a temporal, spatial, or quantitative sense") to have much meaning. Also, because his birth caused his mother's death, Colin and his father do not place much value on his life.
He said it as if he was so accustomed to the idea that it had ceased to matter to him at all.
expressing reproof or reproach especially as a corrective
"Reproach" and "reproof" are synonymous nouns meaning "an act or expression of criticism and censure"--Dr. Craven turns away from Colin, whom he dares not criticize, to reproach, reprove, and censure (all synonymous verbs) Mrs. Medlock, whom he believes should've kept a better eye on Mary and prevented her from finding his secret patient.
marked by extreme intensity of emotions or convictions; inclined to react violently; fervid
"Shall they, Mr. Rajah!" said Mary fiercely. "They may drag me in but they can't make me talk when they get me here. I'll sit and clench my teeth and never tell you one thing. I won't even look at you. I'll stare at the floor!"
The example sentence has both an ironic and pun-filled tone: Mary and Colin are fiercely arguing, not nicely agreeing, but they are agreeable ("conforming to your own feelings or nature") in their identical angry glares.
They were a nice agreeable pair as they glared at each other.
to move in a twisting or contorted motion, (especially when struggling)
"I felt the lump--I felt it," choked out Colin. "I knew I should. I shall have a hunch on my back and then I shall die," and he began to writhe again and turned on his face and sobbed and wailed but he didn't scream.
dignified and somber in manner or character and committed to keeping promises
"Solemn" also means "characterized by a firm belief in your opinions"--both definitions fit the situation because Mary is solemnly examining Colin's back as if she were a doctor, because she wants to prove her solemn statement: "There's nothing the matter with your horrid back--nothing but hysterics!"
Every rib could be counted and every joint of the spine, though Mistress Mary did not count them as she bent over and examined them with a solemn savage little face.