Idiom (n.): An expression whose meaning cannot be deduced from the meanings of its individual words (ex: jump the gun).
I'm going to start including idioms in my GPM posts about commonly confused words. There's nothing wrong with writing these from time to time, but, if you're going to use one, be sure to get it right. :)
Flesh out vs Flush out
To flesh out means to add substance or detail to an object or idea; to develop.
To flush out means to drive or force something out into the open, such as game birds or criminals.