Ha! I was wondering what Quaritch (Col. Coffee Cup) was referring to when he pointed out his head scars to Jake, but never got around to researching it and haven't seen it mentioned here. Then today stumbled on this explanation for a 'shave-tail-louie' at (and other interesting trivia):

"Colonel Quaritch mentions that being on Pandora made him feel "like a shave-tail Louie." "Shave tail" was a term originally used in the 19th century among U.S. cavalry regiments. Newly assigned cavalry troopers were given horses with a shaved tail, to let other troopers know that the rider was dangerously inexperienced, and should be given extra room to maneuver during training. "Louie" is a nickname for lieutenant, the lowest ranking, and least experienced, rank among U.S. Marine Corps officers."