apples and oranges, like comparing

apples and oranges, like comparing

Comparing two unlike objects or issues. This term, dating from the second half of the 1900s, has largely replaced the difference between chalk and cheese, at least in America. The latter expression of disparateness is much older, dating from the 1500s. Why apples and oranges, since they’re both fruits, and not some other object is unclear. Nevertheless, it has caught on and is on the way to being a cliché.
See also: and, apple, compare, like