A handful is the amount that can be held in one hand, or informally, a person or thing that is difficult to control.
From “hand” plus the old suffix “-ful,” meaning “as much as a hand can hold.” This pattern is the same as in “mouthful” and “spoonful.”
The shift from a physical amount to a personality description is clever: a ‘handful’ of a child or problem is just about as much as you can handle at once. English often turns quantity words into character judgments like this.
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