DSsomeone who uses their left hand more than their right hand, especially a pitcher in baseball or a boxer 左撇子;〔尤指〕左撇子棒球投手[拳击手]
Examples from the Corpus
southpaw• Almost certainly the right forefinger, unless Gerry Boden happened to be a southpaw.• Penalosa's lack of mobility is counter-balanced by his southpawstance and hardpunch.