The expression “duck soup” means: Extremely easy; easy as rolling off a log; hence, a cinch.
American slang of some twenty-five years’ standing.
Probably derived from “a sitting duck,” namely one resting on the water, thus easily shot by a hunter; hence, figuratively, an easy mark, any person who lays himself wide open to ridicule or any form of attack.
Thus, to some persons, the solution of a cross-word puzzle or the putting together of a jigsaw puzzle is duck soup.
