Definition from Wiktionary, the free dictionary
Derived from the insignia of an O-1 in the US Military, which consists of a single yellow bar.
- (US Air Force, US Army, US Marines) A 2nd Lieutenant.
- (US Navy, US Coast Guard) An Ensign.
- 1977 -- Rafael Alberto Rivas: Survival: my life in love and war (page 154) 
- "Hey, 'Butter Bar,' " one of the brothers called, referring to the lieutenant, "how much ya want for that round-eyed whore ya got?"
- 2010 -- Green Zone Dir. Paul Greenglass. Perf. Matt Damon. Dist. Universal Pictures.
- "The 101st sent 35 men and one butter bar in here and they don't even have this place secure."