1
general::
Ridiculously easy, as in Setting up a computer nowadays is like shooting fish in a barrel. This hyperbolic expression alludes to the fact that fish make an easy target inside a barrel (as opposed to swimming freely in the sea). [Early 1900s]
American Heritage Idioms
2
general::
Phrase(s): like shooting fish in a barrel [and] as easy as shooting fish in a barrel
Rur. ridiculously easy. • Jane’s a good mechanic. Changing a tire is like shooting fish in a barrel, for her. • That comedian has an easy job. Making fun of politicians is like shooting fish in a barrel.
McGrawhill's American Idioms And Phrasal Verbs