Anything easily done; cinch, piece of cake: It would be duck soup for him to have led Willis into a trap/ Only duck soup for a guy like you(1908+)
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 duck soup
An easily accomplished task or assignment, a cinch to succeed, as in Fixing this car is going to be duck soup. This expression gained currency as the title of a hilarious popular movie by the Marx Brothers (1933). The original allusion has been lost.
[ Early 1900s