"Lefty" is a Terran Dominion marine. At some point in his service, his CMC Powered Combat Suit was compromised, and it locked down and sealed off his damaged arm to secure the suit, severing his right arm. He later served as an example as to why marines wanted to avoid a suit lockdown.
Barba, Rick. StarCraft Field Manual (hardcover). Insight Editions, November 17, 2015.