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 bottom of the barrel
The least desirable, the dregs, as in The nominating committee had trouble finding candidates; they were settling for the bottom of the barrel. The phrase often occurs in
scrape the bottom of the barrel, meaning “to use the least desirable elements” (because one has no choice), as in Bringing up that minor legal point proves that you're scraping the bottom of the barrel. This metaphor for the sediment left by wine in a barrel was already used by Cicero to describe the lowest elements of Roman society.
[ First half of 1900s