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Phrase(s): *knee-high to a jackrabbit [and] *knee-high to a grasshopper
Rur. very small or short. (Usually used to describe children. *Typically: be ~; since someone was ~.) • I’ve known you since you were knee-high to a jackrabbit. • My, how you’ve grown! The last time I saw you, you were knee-high to a grasshopper!
McGrawhill's American Idioms And Phrasal Verbs