The word is abbreviated from Boondock, American military slang for the Tagalog word 'Bundok' picked up during the Phillipine-American war (the Tagalog people make up a quarter of the population of the Phillipines).
A boonie hat or bush hat has a wide brim, often bucket-style hat that is typically worn for protection from the sun and other elements. They are made from a variety of different material, including canvas, cotton, or polyester. Some boonie hats also feature vents or mesh panels to help keep the head cool.
Originally designed for military forces, they were made so army men and women could have better protection while in the jungle and were often made with camouflage patterns and in an earthy colour. The brim of the hat helped to keep the sun and rain off their face, while the soft material prevented any kind of insect from biting them. They would have branch loops on the crown so that camouflage could be added in the form of additional vegetation, such as jungle foliage. They came into world prominence when given to the US army during the vietnam war where fabric tape band was added to hold additional vegetation.