The old expression in my title here—close cousin of ‘you could knock me over with a feather‘—is a colloquialism for great surprise. It’s also, was so many colloquial phrases are, comically colorful and exaggerated. Hyperbole and humor grow out of genuine life experiences and events. They are anchored in recognizably real sensations and situations as much as in silly imagination.
After all, when in shock, don’t I feel my knees suddenly go uncomfortably loose and slack, and go all light-headed as though not only my hat but the whole top of my head might just fly off at the slightest whiff of a breeze?
The appeal of the idiomatic phrase, regional quirks of language, colloquial expressions and other ‘decorative’ forms of description is that they can often communicate far more about the situation than mere facts can do. They share more of our personality and character and our own experience of the moment than the dry recitation of what can be scientifically charted could ever be able to convey. And they amuse and entertain, which gives much more impact to any statement than simple and bald recitation, and what has such verve is more likely to stick in the memory as well.
Also, it’s kind of hard to make a little illustrative sketch that conveys the astonishment with which an unforeseen hat-snatching or similar surprise nearly so well as one that says, Well, Blow Me Down! If you know what I mean.