The term swabbie is a nickname for naval personnel used by UNSC marines and ODST Troopers. Because the Spartan-II's are technically a part of the UNSC Navy, they also qualify as swabbies. The term refers to the old tradition of "swabbing the decks," where the crew would clean the decks of old wooden ships.


Quotes

"Last time I checked, I take my orders from the Corps, not from some swabbie...sir"
Corporal Locklear

"You'd better get it together, Marine, or the Chief will reach down your cornhole and pull you inside out...which will be a sweet, sweet mercy compared to what I'm gonna do to you"
Sergeant Major Avery J. Johnson


Sources

Halo: First Strike