Despite the association with what happens to any toilet on a regular basis, it is not unusual to see people “flush out an idea.” Some ideas certainly deserve such treatment, but the phrase is “flesh out an idea.” Or flesh out the details, story, plan, etc.
It’s an odd concept, to add flesh to something intangible, like an idea. Flesh as a verb is first recorded in the 16th century with various meanings related to giving your hunting bird a bit of the kill or plunging a sword into someone (“Don’t make me flesh you.”)
In 1661, Thomas Fuller wrote, “This bare Sceleton of Time, Place, and Person, must be fleshed with some pleasant passages,” adding figurative flesh to a concept for perhaps the first time.
We’ve only spoken of flushing with water since the late 18th century, and the origin of that word is unclear. We earlier spoke of a flush, as a noun, as a sudden increase in water, as with a flood. It’s also an adjective that commonly described the fullness of a stream and now can refer to having plenty of something. But flushing may be applying to liquid what happens with birds and butterflies when they quickly take flight. If we use flush out to mean drive an animal from a bush or an escaped convict from a cabin in the woods, we’re using the nonliquid meaning first recorded in the 14th century.
Adding to our confusion, perhaps, is that flesh can become flush. That reddening of the cheeks seems to be related to flush as bountiful. But the concept also may be influenced by the idea of blood flowing to the face.
The bountiful sense of flush might also add to confusion with flesh. We can be flush with ideas, too.
And then there is the flush that describes two things level with each other. The Oxford English Dictionary suggests that relates to that full stream, as the water runs flush with the top of the bank. But its first citation, from 1791, refers to buildings and not rivers.
With all the meanings of flush and flesh, its not surprising that these two make us pause and sometimes err. Remember that the verb form of flush means to clear something with liquid, get rid of something, or drive something out into the open. The verb form of flesh, unless you hunt with a hawk, now only means to add substance to something, and that’s a positive goal for our ideas.