"The Farmer in the Dell" is the song that Omar Little whistled in the U.S. series "The Wire" to let people know he was coming. When this was done, everyone ran away so that drug dealers targeted by Omar were left alone to be robbed. Omar whistles the song as he strolls through the block -- people then run and hide from him as he looks around and selects his target (they are the guys who notice him last and are slowest to react).

Omar Devone Little, a fictional character on the 2002–2008 HBO crime drama series "The Wire", was portrayed by Michael K. Williams. He was a notorious Baltimore, Maryland stick-up man, who often robbed street-level drug dealers. He was legendary around Baltimore and was known for wearing his characteristic duster, under which he concealed a shotgun, large caliber handgun, and bulletproof vest. People could not mistake him because he was known for his facial scar and his whistling of the song "The Farmer in the Dell" when he stalked all of his local street victims.

More Info: en.wikipedia.org