To make or compose something, esp to write something, in a hurry; cobble up (1940s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with bang out
Produce something loudly or hastily by striking, either a musical instrument or a typing keyboard. For example, The accompanist banged out the melody on the piano, or John planned to bang out his presentation in a couple of hours.
[ Late 1800s