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 crack-dawn
Very early morning, daybreak. For example, I got up at the crack of dawn. The crack in this term alludes either to the suddenness of sunrise or to the small wedge of light appearing as the sun rises over the horizon. Originally the term was usually put as
crack of day.
[ Late 1800s