I was reading Suzanne Brockmann's novel The Defiant Hero, and I came across this phrase:

as they crossed the roof on their bondoons.

I had no idea what bondoons were. My dictionary didn't list the term. But a quick web search revealed the answer, in a thread about sniglets. One of the thread participants had emailed Brockmann about the word, and Brockmann had replied:

It's actually made up word—a euphemism for one's bottom that my husband created when our kids were little.

It actually first came to be as “bondoony.” Which is a somewhat silly word that made everyone laugh—especially the two-year-olds who often fell on their bondoonies.

It turns out there are a couple of other instances of the term on the web, most notably a thread from a system optimization forum from 2000:

Now there's a large pain in the bondoons.

The reason that's notable is that the book came out in 2001; presumably other people had picked up the word from Brockmann and her family before the book was published.

Google also gives a search result that includes the phrase “sitting at home on our fat bondoons,” but the article that that allegedly appears in doesn't actually contain that phrase, and Google's cached copy says the term appears only in pages linking to that page. Odd and confusing.

Anyway, pleased to be able to find the answer to the mystery so easily; thanks to the person who emailed Brockmann for asking and posting, and thanks to Brockmann for responding to that question.