What does pants-dropping mean?
A pants-droppingly good film.
About the orphan kid who goes on to become pantsdroppingly wealthy.
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As you already know, "jaw dropping" simply means astonishing or amazing. Example: "The piano solo was jaw dropping!"
"Pants dropping" is not really idiomatic at all. If it was used, it would make most sense if said by a female, and would mean she found someone very sexually attractive. Example: "Did you see Brad Pitt in that movie? He was gorgeous -- just plain pants dropping". There is a rough example of this in an episode of "How I Met Your Mother" where the girl Robyn expresses her fondness for the players of the Vancouver Canucks ice hockey team (although now I think about it, she says "panties" not "pants")
And, for completeness: "Dropped my pants" has a completely different meaning. It deploys gay sexual connotations to express one putting oneself in a vulnerable position. Example:
Jim: I really wanted to win the sale, but he said the price was too high
Bob: So did you just walk away?
Jim: Walk away? Hell no; my boss would have killed me! I just dropped my pants and gave him what he wanted.
It is slightly ridiculous, and very informal - I might use "jaw-droppingly" in conversation with a superior, but never "pants-droppingly". It was never meant to be a serious description of something; it's an exaggerated and somewhat humourous way of saying that something is very good.
In other words, the ridiculousness of it - the fact that it makes no logical sense - is part of what makes it effective as a phrase.
Expressions like "No pants!" or "I dropped my pants!" often have sexual connotations, especially when exclaimed by young men via the internet. The idea is that something (like the awesomeness of their favorite sports team) is likely to make them ecstatically blissful, in such a way that they would make a mess in their pants (if they were wearing pants). So they have decided not to wear pants.