Why the hell this movie called "Southpaw" ?

Rating: 7/10 (1 September 2019)

Pretty cool for boxing movie but why the name is southpaw? The guy was not southpaw lol.
For those who don't know what is southpaw it's term for left handed boxers like Rocky Balboa. But this guy is right handed so wtf? ðŸ˜‚