From BR Bullpen
LOOGY (Left-handed One Out Guy) is a typical modern role for a left-handed relief pitcher, who comes in to face just one left-handed batter or two. The canonical example of a LOOGY was Jesse Orosco: for the last 13 years of his career, Orosco never averaged more than 1 inning pitched per appearance in any season, and in the last 5 years of his career he averaged less than 1/2 IP per appearance. Others who have had long careers performing this role include Mike Myers, Trever Miller and Will Ohman. They will typically have a lot of appearances, few innings pitched, and very few decisions - wins, losses, or saves.
The term was coined by the staff of the on-line magazine Baseball Prospectus in the early 2000s, when the number of such pitchers proliferated. Tony LaRussa was known as a manager who was excessively fond of this type of pitcher, always keeping at least two and sometimes three in his team's bullpen, and making more in-inning pitching changes than any other manager.