not cricket, it's/that's

it's not cricket

It isn't fair, sportsmanlike, or legitimate. I know you want to avoid confrontation, but it's not cricket to break up with someone by text message. I don't see why you think it's not cricket—everyone else does it all the time.
See also: cricket, not
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

It's not cricket.

 and It's not kosher.
It's not done.; It's not acceptable. You can't do that! It's not cricket!
See also: cricket, not
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

it’s (just) not ˈcricket

(old-fashioned, British English, informal) it is not a fair or an honourable action or way of behaving
See also: cricket, not
Farlex Partner Idioms Dictionary © Farlex 2017

cricket

mod. acceptable. (See negative examples at not cricket.) Is it really cricket to play under two different names?
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.

not cricket, it's/that's

Unsportsmanlike, unfair, dishonorable. Eric Partridge traced this term to 1867 but believed it was not widely used until the early twentieth century. Among the early references in print is Stanley Houghton’s 1914 play, The Partners, “. . . but it is not playing the game. In other words, Cynthia, it is not cricket.” Although cricket is a sport popular exclusively in Great Britain and most of its former colonies, the term crossed the Atlantic and became a cliché in the United States as well.
See also: not
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
See also: