the mind > mental capacity > perception or cognition > faculty of imagination > inventive or creative faculty > [adjective] > of one's own invention (3)
of one's own study: (of a text, etc.) of one's own composition. Obsolete. rare.
out of a person's head: from a person's mind, imagination, or invention. Often in the more emphatic form out of a person's own head. Frequently…
Devised or invented by oneself.