(n.) a fantastical, short-lived idea or whim
A brilliant idea that suddenly comes to you, but then, after a few moments of thinking or doing, turns out to be not all that great an idea? That’s a moonflaw.
Or at least, that’s one definition of a moonflaw. When that word first appeared in the English language in the 1600s, it was a decidedly more serious affair: back then, a moonflaw for a temporary derangement or a loss of sanity, the kind that was once widely held to be caused by the Moon’s inexplicable effect on our bodies. (Hence, of course, the word lunatic.)
Because such lunar flights of fancy were understood to be resolved by the following morning or the passing of a full moon, they were short-lived and only temporary states—and hence use of the word appears to have weakened in some contexts, to eventually come to refer to any short-lived idea or whim.