To flesh out an idea means to add details, examples, and substance to make it fuller and more complete. When you flesh out a story outline, you transform bare plot points into scenes with descriptions, dialogue, and character development. When a scientist fleshes out a hypothesis, she adds supporting evidence, explanations, and data to make her basic idea convincing and thorough.

A skeleton shows the basic structure, but flesh makes it complete and real. Similarly, a fleshed-out plan isn't just “build a treehouse” but includes what materials you'll need, how you'll attach the platform, where the ladder will go, and how you'll make it safe.

Teachers often ask students to flesh out their rough drafts by adding specific examples, expanding on their reasoning, or including more sensory details. A book report that says “The character was brave” needs fleshing out with examples of what the character actually did to show bravery. The difference between a sketch and finished work is often how much you've fleshed out the details that bring your idea to life.