Perhaps you already know what “claptrap” originally meant. I hadn’t.
I thought it meant more or less the same thing as “bunkum” or the less polite “bullshit”: content-free or transparently insincere rhetoric: “That’s just a bunch of claptrap.” The image I had in mind was of someone’s “trap” (mouth, as in “Shut your trap!”) clapping aimlessly, more or less the same image as “just flapping his jaw.”
Not so. Macaulay, writing in the mid-nineteenth century, refers to “clap-traps” (hyphenated, and in the plural) as meaning “standard arguments sure to draw applause from partisans.” He refers to the losing side in some dispute as “still having a few reliable clap-traps,” and then goes on to list the “clap-trap” arguments. So a “clap-trap” was originally something close to the contemporary “applause line.”
It’s not hard to see how the transition from the meaning “applause line” to the meaning “bullshit” could have happened, since it’s frequently true that the arguments most likely to cause a partisan crowd to clap don’t actually hold water. Still, a somewhat surprising (to me) bit of linguistic history.
As for why I’ve had both meanings of the term on my mind, I’m attending the California State Democratic Convention. See the post immediately above.