The fascine knife was a side arm / tool issued to 17th to 19th century light infantry and artillery. It served both as a personal weapon and as a tool for cutting fascines (bundles of sticks used to strengthen the sides of trenches or earth ramparts protecting the batteries). It could be straight or curved, double edged or single edged with a sawtoothed back. 17th and 18th century German, Prussian and Swedish fascine knives are more like calvalry swords, often with a brass handle and a hand guard, but later models are more like billhooks in shape and appearance. By the 20th century in the British Army it had become the Pioneer's billhook, used in WW1 for making machine gun emplacements. In the Indian Army it is known as a Knife Gabion (gabions, like fascines, are used for supporting earthworks).
Some types of fascine knife are probably descended from early weapons like the baselard and the großes Messer. Others, known to British foot soldiers as a billhook, are more closely related to agricultural cutting tools used for over 2000 years throughout Europe and also independently developed in many other countries (e.g. the billhook was used in China and Japan before western explorers visited the country). Like the billhook they were used for cutting saplings (e.g. willow, hazel or chestnut) that were bundled up to make fascines or woven into hurdles, or gabions. Many revetments used a combination of all three, with fascines at the bottom of the trench, hurdles just below ground level and gabions above, filled with the earth from the trench.
Many so called 'fascine knives' currently offered for sale in the United States are, in fact, agricultural tools that have never seen military usage, although historically many billhooks probably became used as weapons by their conscripted owners. Although the Spanish Army called its fascine knives machetes, they bore little resemblance to the common cutting tool.