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It's in reference to a class Modern Warfare 2,made of of your choice of primary and secondary weapons and Marathon Pro, Lightweight Pro, and Commando Pro as your perks. Most commonly, your secondary would be a pistol with the tactical knife attachment. A Happy Runner runs around knifing people and risking the death count for kills.
Ranger is a Happy Runner on Modern Warfare online, she racks up knife kills by the second!
by ResistanceRanger March 13, 2010