[1922+; fr the count of ten made over a downed boxer]
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 down for the count
down for the count
Defeated, finished, doomed, as in I doubt if he'll ever leave the hospital; his illness really has him down for the count. This expression alludes to the losing boxer, unable to get to his feet before the umpire has counted to ten (the count).