southpaw vs left-hander

southpaw and left-hander both are nouns.

Word	Noun	Adjective	Verb	Adverb
southpaw	Yes	No	No	No
left-hander	Yes	No	No	No

As nouns, southpaw and left-hander are synonyms defined as:

southpaw and left-hander: a person who uses the left hand with greater skill than the right

Other synonyms of southpaw include lefty.

As nouns, southpaw and left-hander are synonyms defined as:

southpaw and left-hander: a baseball pitcher who throws the ball with the left hand

Other synonyms of southpaw include left-handed pitcher, left hander, lefthander, lefty.

southpaw (noun)	left-hander (noun)
a person who uses the left hand with greater skill than the right	a person who uses the left hand with greater skill than the right
a baseball pitcher who throws the ball with the left hand	a baseball pitcher who throws the ball with the left hand