Wordorigins.org

A southpaw is a left-hander or the left hand. Today, the word is primarily used in baseball, but appears in other contexts as well. But despite its use in baseball, the term almost certainly did not originate with that sport. Instead, it is more likely from the world of boxing.

The earliest known use of southpaw is in a political cartoon from 1848 titled Democratic B-Hoy (boy). The cartoon depicts Democratic presidential candidate Lewis Cass in a brawl with Zachary Taylor and a number of other Whigs. In the cartoon Cass is winning the fight, although in actuality he lost the election to Taylor. One of the Whigs, who has been knocked to the ground, says:

Curse the Old Hoss, what a south paw he has given me!

There are numerous other citations of use in the mid-to-late 19th century, often with southpaw as a nickname for a man, who is presumably left-handed. The south is most likely simply a reference to the opposite orientation than is usual and the paw is self-explanatory.

It is commonly claimed that the word originated in baseball slang, although given the quote above it is clear that boxing is a more likely source. And with a date of 1848, when the game of baseball as we know it today was in its infancy, baseball just does not appear to be a plausible source. Use of the word southpaw in reference to baseball is not known prior to 1885. From Sporting Life of 14 January 1885:

They had always been accustomed to having their opponents hug their bases pretty close, out of respect for Morris’ quick throw over to first with that south-paw of his.¹

Those who claim a baseball origin explain the term by the “fact” that 19th century baseball diamonds were often arranged so the batters would face east, to avoid looking into the afternoon sun. The pitcher’s left hand, or paw, would therefore be on the southern side. But this explanation, while plausible in and of itself, does not agree with the lexical evidence, which puts baseball usage decades later than that of other contexts.

¹Oxford English Dictionary, southpaw, n. (a.), 2nd Edition, 1989, Oxford University Press, accessed 1 Jan 2009 <http://dictionary.oed.com/cgi/entry/50231743>.