"Lax" also means "without rigor or strictness"--despite threats to whup Ma and the younger children, Pa is shown mostly as a loving husband and father who tries his best to provide for his family; his attitude toward discipline could be described as lax. Here in the example sentence, his hands, which are normally strong and firm from manual labor, are lax because he's helplessly surprised at Ma's sassy revolt.
The whole group watched the revolt. They watched his lax hands to see the fists form.
the feeling that accompanies something extremely surprising
In addition to wielding a weapon, Ma scolds, "What we got lef’ in the worl’? Nothin’ but us. Nothin’ but the folks." This fierceness astonishes her (Latin root is "tonare" which means "to thunder"), yet vividly emphasizes the lengths a mother would go to in order to keep her family together and safe.
the feeling that everything is wrong and nothing will turn out well
And it might be that a sick child threw despair into the hearts of twenty families, of a hundred people; that a birth there in a tent kept a hundred people quiet and awestruck through the night and filled a hundred people with the birth-joy in the morning.
The families learned what rights must be observed—the right of privacy in the tent; the right to keep the past black hidden in the heart; the right to talk and to listen; the right to refuse help or to accept, to offer help or to decline it; the right of son to court and daughter to be courted; the right of the hungry to be fed; the rights of the pregnant and the sick to transcend all other rights.
And the families learned, although no one told them, what rights are monstrous and must be destroyed: the right to intrude upon privacy, the right to be noisy while the camp slept, the right of seduction or rape, the right of adultery and theft and murder.
"Prospect" also means "someone who is considered for something"--the "someone" here is a young man and the "something" is marriage; this is supported by the linking of the words "prospects" and "popularity" and the setting of the novel in the 1930s, when the possibilities for women succeeding outside the home were slim.
And young girls found each other and boasted shyly of their popularity and their prospects.
"Abruptly" comes from the Latin "rumpere" which means "to break"--although the family was already falling apart with the death of Grampa, Noah's abrupt desertion creates a deliberate rupture in the family. But traveling with the other migrants on the same road introduces the Joads to a new vision of family.
“It ain’t no use,” Noah said. “I’m sad, but I can’t he’p it. I got to go.” He turned abruptly and walked downstream along the shore.
"Flighty" also means "guided by whim and fancy"--this definition is the opposite of Ma's practical nature: her flightiness here is mostly nervous anxiety that the inspection officers might stop them on account of Granma; it could also be a pun on her focus on the family's flight across the desert to California, where they can start a new life.
Tom said, “I don’ know what’s got into Ma. She’s flighty as a dog with a flea in his ear.
And whereas the wants of the Californians were nebulous and undefined, the wants of the Okies were beside the roads, lying there to be seen and coveted: the good fields with water to be dug for, the good green fields, earth to crumble experimentally in the hand, grass to smell, oaten stalks to chew until the sharp sweetness was in the throat.
And such a man drove along the roads and knew temptation at every field, and knew the lust to take these fields and make them grow strength for his children and a little comfort for his wife. The temptation was before him always. The fields goaded him, and the company ditches with good water flowing were a goad to him.