To be knee-high to a grasshopper means to be very short or very young. Though the second meaning is heard more commonly. The idiom literally means to reach a grasshopper’s knee. It is usually used in reference to a time long ago when someone was younger/littler than the present.
The first term is always hyphenated as an adjective describing someone or something’s height.
History
This idiom originated in the United States in 1814 as the phrase knee-high to a toad. Many animals have been used in the idiom, such as mosquito, duck, jackrabbit, bumble bee, splinter, and others. It was about 1850 when grasshopper was introduced into the phrase in a political opinion article describing the opposition. Over time grasshopper has become the standard comparison.
Examples
When I interviewed Judge Judy, I realised that I was interviewing an icon, which was quite nerve-wrecking. She was really lovely – she’s only knee-high to a grasshopper, and she’s a bundle of energy. [Irish Independent]
I’m sure they didn’t know who I was, in fact, I doubt if they noticed me, but when I got within hearing distance, a little bloke who was about knee high to a grasshopper yelled out at the top of his voice, “Look out here comes Meckiff!” [Mid-day]
“I have known Mitch since he was knee-high to a grasshopper and it’s no surprise to see him putting it together. At times we can be impatient, but the kid is only 22, and he’s missed a lot of cricket because of injury,” Moody said. [Sydney Morning Herald]