To be short, usually in the context of also being young.
The first known recording of this expression was in the USA in 1814 as knee-high to a toad. Other forms include frog, bumblebee, splinter, mosquito, jackrabbit and, of course, grasshopper which can be found in The Democratic Review in 1851:
"You pretend to be my daddies; some of you who are not knee-high to a grasshopper!
More expressions and their source
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Theo of Golden by Allen Levi
One spring morning, a stranger arrives in the small southern city of Golden. No one knows where he has come from…or why…
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