Ben Yagoda notes that "a disproportionate number of all idioms are nonsensical, or, at the very least, don’t parse":
That’s why they’re idioms. Take (randomly) Be that as it may. We all know the idea it conveys. But exactly how (if at all) do the words in the phrase combine to mean that? I’m sure some of you smart people out there can answer the question. I cannot. And that’s no big deal. Sometimes idioms start out as sensical and make a transition to non-, and it’s fun to chart their progress. One I have my eye on at the moment is can’t help but, as in I can’t help but think that Philip Roth is going to win the Nobel Prize. It’s easy to see how it emerged—as a new, improved combination of two older idioms, I cannot but think that … and I can’t help thinking that. … The grafted version may be strictly speaking nonsensical, but it’s got rhetorical punch and has had remarkable success in taking the place of its parents.
The Dish 