...Interesting. I just did a spellcheck on the word bouncy, and while syllabub does not get the yellow marker, negus does. Strangely, "Jello-y" gets a pass as well...
Not surprisingly, it's a dessert. There are many variations on the theme, but the basic idea seems to be a heavy milk or cream mixed with sugar and slightly curdled by the mixture of wine or some other alcohol. It is supposed to have been in fashion between the sixteenth and nineteenth centuries, though really I can't understand why it would ever go out of fashion. Cream, alcohol and sugar--what's not to like?
There are many, many recipes out there on line, but without the slightest intention of ever making syllabub, the one cited on Wikipedia is my favorite. It says to mix the other ingredients together in a large bowl, "place the bowl under the cow, and milk it full."
Rather disappointingly, the etymology is unknown, and no one that I found even ventures a guess. The OED says that "sillibus" is the preferred spelling, which I suppose will come as a bit of shock to the Baroness James. I was hoping that there would be a link to rather similar words like syllabus and syllable, but no one suggests it. I was curious to find that "syllable" actually means "several sounds taken together", and wonder if "several things taken together" might be part of the association here. Of course those are the kinds of guesses that true etymologists frown upon.
While wandering around looking for information, I ended up on the Dictionary.com site, on the word syllabus. Ever hopeful of extending our limited vocabularies, they have a little quiz at the end. This time it started out:
"Syllabus is a great word to know. So is search warrant."
Yeah, that could come in handy.