"That's Just Not Cricket"

Meaning:

When someone asks you to do something or something happens to you that you think is not fair.

Origin:

This originated within the game of cricket. In February 1st 1981, Australia played New Zealand in the World Series Cup. In the final over Trevor Chappell was bowling for Australia against New Zealands, who needed 6 runs off the last ball to tie the match when Chappell controversially bowled an underarm bowl making it impossible for the New Zealands batsman to hit a 6. This was dubbed "the most disgraceful moment in cricket" and the term "It's just not cricket" certainly applied here.

Below: Trevor Chappell's Underarm Bowl