southpaw vs lefthander

southpaw and lefthander both are nouns.

Word	Noun	Adjective	Verb	Adverb
southpaw	Yes	No	No	No
lefthander	Yes	No	No	No

As nouns, southpaw and lefthander are synonyms defined as:

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

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

southpaw (noun)	lefthander (noun)
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