All is fair in love and war
Meaning: Everything can be justified between people who love each other and times of war.
History of the phrase: This phrase, which is now a very common saying, dates to 1578. It first appears in John LyLy’s, ‘Eupheus: The Anatomy of Wit‘.
Synonymous Expression: Do whatever it takes!
The opposite of all’s fair in love and war is fairplay which means to have be a good sportsman (i.e. a person who is fair when playing a sport).