1. very young and, therefore, very small

1. Mr. Smith always reminds me that he knew me when I was just knee-high to a grasshopper.

2. I haven't seen you since you were knee-high to a grasshopper.

3. My, how you’ve grown! The last time I saw you, you were knee-high to a grasshopper!

4. I’ve known you since you were knee-high to a jackrabbit.