This excellent word (“Origin unknown”) is best explained by quoting the OED’s citations:
1936 Allen & Lyman Wonder Bk. Air 312 A modoc, the derivation of which is obscure, is a flashy chap who goes around wearing helmet and goggles, and more than likely, leather boots and riding breeches, too, and talking about the big things he is going to do for aviation. 1942 Berrey & Van den Bark Amer. Thes. Slang §756/2 Modock, one who has taken up aviation for publicity, social, or similar reasons. 1960 Wentworth & Flexner Dict. Amer. Slang 341/2 Modoc, one who becomes an Air Force flier for publicity, social prestige, or similar reasons.
As far as I can tell by googling, the word is dead as a mackerel, which is a pity—it has a fine slangy ring to it. (Found at The Sensible Ass, a blog which makes a habit of listing odd and interesting words.)
Update. According to Mike (in the comments), modock (as he spells it) “is currently used by a large group of U.S. pilots”; I am happy to retract my statement that it was dead as a mackerel. Mike is interested in the early history of the word, so if you know anything about it, please e-mail him (click on his name in the comments for the address).