Or an example of the relative height of maize as compared to the lower extremities of able bodied human beings on the fourth day of the seventh month of the Gregorian calendar.
The wife & I, trying to remember any semblance of normalcy, drove all around the area farms yesterday looking for knee high corn, given the titular colloquialism. To our surprise, we discovered most of the corn was already way past chest height.
After almost giving up on the concept, we found this comparatively stunted crop in a nearby community garden.
The image itself is kind of an inside joke. I have a habit of butchering colloquialisms. Such as “six of one; half dozen of another” in regards to equal quantities, becomes “half of one; six dozen of another,” but still used in the original capacity in a manner of attempted ironic humor.
“Knee high by the Fourth of July” entered our lexicon as “ankle deep on a rainy Thursday in the third week of April” or “hip length on the 17th of June” or “up to your armpits in August” and other such variations of nonsensical meaning.
My wife & I often bat these pseudo-sayings around without regard for our audience, sometimes leading to semi-awkward explanations, akin to the one you’ve just read.