Struck with amazement; thunderstruck; "moon-struck"; Shropshire.--Joseph Wright's English Dialect Dictionary, 1896-1905
Death of John Dennis (1657-1734), English playwright and critic, who in 1709 devised a method of simulating thunder onstage with a large mustard bowl for his dramatic work Appius and Virginia. Despite this well-crafted special effect, the play closed quickly. The Drury Lane Theatre manager admired the thunder-generator, using it soon afterward for a production of Shakespeare's Macbeth, which Dennis attended. Hearing a distinctive thunderclap and realizing that his mechanism was responsible for it, he purportedly coined a common expression on the spot: "See how the rascals use me. They will not let my play run, yet they steal my thunder!" In The Dunciad (1728) Alexander Pope included this mock tribute to Dennis:Let others aim, 'tis yours to shake the soul,
With thunder rumbling from the mustard-bowl.
I assume here that "Shropshire" is where the term originated, rather than being another meaning for 'comic struck.' ("I'm amazed! I'm astounded! I'm Shropshire!") Also, I apologize for the quotes being inside the semicolon rather than the other way around, but that's how it was in the original text.
-The Gneech, just borrowing somebody else's thunder for a few minutes with every intention of returning it