affected with or marked by frenzy or mania uncontrolled by reason
"Lashing" is a "beating with a whip or strap or rope as a form of punishment"--this word and definition almost personifies the wind, which is supported by Charles Wallace's observation that "if I concentrate very hard I can understand the wind talking with the trees." But to Margaret (Meg), the wind is scary because it is a frenzied force lacking reason; worse, it seems to echo the direction of her own life and emotions.
In her attic bedroom Margaret Murry, wrapped in an old patchwork quilt, sat on the foot of her bed and watched the trees tossing in the frenzied lashing of the wind.
"Fury" also has the "property of being wild or turbulent"--this would fit the nature of a high school teenager who's unhappy with the way she looks, upset that she is in danger of being left back in school, tearful because her father is missing, and willing to let her anger take on an older and heavier boy in a fight that results in her having a torn blouse and big bruise under one eye.
Meg would turn white with fury when people looked at him and clucked, shaking their heads sadly.
The furnace purred like a great, sleepy animal; the lights glowed with steady radiance; outside, alone in the dark, the wind still battered against the house, but the angry power that had frightened Meg while she was alone in the attic was subdued by the familiar comfort of the kitchen.
having or emitting a high-pitched and sharp tone or tones
The physical quality of Meg's voice at this moment is close to wailing. "Shrill" also means "being sharply insistent on being heard"--while Mrs. Murry hears Meg's protests, she is still going out in the storm to see what their dog is growling about. This makes Meg shrilly insist on going with her mom, because she doesn't want to end up losing another parent.
"Ferocious" is not an adjective often used to describe braces, especially since braces are meant to straighten teeth to make them pretty instead of sharpening them to make them dangerous. The ferocity of the braces comes less from the barbed lines of wires and more from the way Meg is deliberately using them to reveal her anger at the principal's questioning of her father's occupation and whereabouts.
Meg bared her teeth to reveal the two ferocious lines of braces.
"Peremptory" also means "offensively self-assured or exercising unwarranted power"--this could describe the nature of a five-year-old boy who both interrupts an adult while she's speaking and scolds her for stealing without consulting him first. But Charles Wallace's age actually makes him seem less offensive and his "uncanny way of knowing" gives him a powerful assurance that is scarily unusual.
But Charles Wallace held up his hand in a peremptory gesture.