His thesaurus is broken. That's a terrible word to use to describe an opinion in 2008. My O.E.D. give a definition that works well in this context, "unstable or unsettled in opinions, etc.; inconstant; apt to change quickly," but then offers that this meaning was "Frequent in the 17th century" and offers an 1896 citation as its most recent usage. Otherwise it just means "steep" or "vertigo-inducing." There have to be a dozen better words to describe an excessively malleable opinion than that one. Assuming that your primary goal is to communicate to your audience, of course, and not to emulate George F. Will.