1
general::
Phrase(s): flesh out
to become more fleshy. • She began to flesh out at the age of thirteen. • After his illness, Tom fleshed out and regained his strength., Phrase(s): flesh something out (with something)
Fig. to make something more detailed, bigger, or fuller. • This is basically a good outline. Now you’ll have to flesh it out. • The play was good, except that the author needed to flesh out the third act. It was too short.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, put flesh on the bones of. Give substance to, provide with details, amplify. For example, The editor told her to flesh out the story, or You need to put flesh on the bones of these characters. This metaphoric expression, alluding to clothing a nude body or adding flesh to a skeleton, was in the mid1600s put simply as to flesh, the adverb out being added about two centuries later.
American Heritage Idioms