duck soup

Fig. very easy; an easy thing to do. For Maria, knitting a sweater is duck soup. Jill: This jar is stuck. Could you open it for me? Jane: Sure. 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]

Easy to accomplish. The first appearance of the phrase was in a 1902 newspaper cartoon that had nothing to do with ducks. Not then and not now has anybody been able to suggest a likely derivation. If you're interested in an expression that makes sense, try the equivalent, “as easy as falling off a log.”