When I first started playing halo online, I played as MotorKiller. That's the nickname for my rating in the USCG. Machinery Technician was abbreviated (don't ask me why) as "MK." We are jokingly referred to as "MotorKillers" as we are always fixing (breaking?) engines on the Coast Guard boats.
Later on, I was playing as Maddogmedic when I was recruited (briefly) into the WNx clan where I played as WNx_Maddog. (maddog was my personal nickname when I was in the military).
When I started racing, I went back to the Motorkiller name and then EtOH then bfm_EtOH. It was so hard to pronounce on Vent that I went to 'Medic as a short version of Paramedic.
Why 'Medic/Paramedic, you ask? Well, um, it's kind of complicated: It's because I'm a licensed paramedic. Crazy, I know, but that's what I do! I also teach in a paramedic baccalaureate program at a university on the East Coast of the US.
I get a lot of guests in our servers asking me if I'm really a Medic and I always answer, "yep!"
-'Medic