“Knee-high to a grasshopper” is an informal expression for someone who is very young or sometimes very small. Most often, it refers to childhood. People usually say it with a warm, humorous tone when remembering someone as a little kid.
The image is intentionally exaggerated. A grasshopper is already tiny, so saying a child was only as high as a grasshopper’s knee creates a playful picture of extreme smallness. The phrase is strongly associated with rural American speech and older storytelling styles, which is why it often sounds folksy or affectionate.
You will usually hear it in sentences about the past, especially with when or since.
- I have known Maya since she was knee-high to a grasshopper.
- He wanted to drive a tractor when he was knee-high to a grasshopper.
In modern use, the phrase is not technical or formal. It is better in conversation, memoir, and friendly writing than in business or academic contexts. If you want a plainer alternative, you could simply say very young or as a small child. The idiom survives because the image is vivid, funny, and easy to remember.