"Frag" is a short for "fragmentary order
', the piece of a larger order for an operation that unit is expected to execute. When most military units get an order, what they're really getting is a frag because what they're doing is almost always part of a larger operation. As armies can be quite large, co-ordinating the many parts of a large military force is quite complex.
Let me offer an example. During the Second World War, most American armored divisions were the triangular divisions, meaning they had three tank, three armored infantry, and three mobile artillery battalions, plus associated headquarters, medical, maintenance, communications, quartermaster and other support subunits. The unit had around 272 tanks, 452 half-tracks. and fifty-four M7 Priest self-propelled guns, and 1,869 vehicles of all types, and 10, 670 soldiers when at full strength. American fully tracked tanks and SPs were very reliable for the type, but all tracked vehicles require constant maintenance and a road march means lots of breakdowns, and the division drinks seventy-four tons of fuel every day. In combat a unit that size can easily consume a thousand tons of supplies every single day. Division vehicles alone fully occupy three miles of roadway at any one time. In a large operation, other subunits of other divisions will probably also be moving, and in any particularly area the road net can be quite constricting. The reason Bastogne became a battlefield during the Battle of the Bulge is because most local highways ran through Bastogne. If the German offensive was to succeed many military subunits and their supplies would have to pass through Bastogne, making it critical ground.
So to move our theoretical armored division to Bastogne you must first tell each of the many major subunits where they need to go, when they need to depart, when they need to arrive and what road they are to take. Moving different units down parallel roads makes the unit less vulnerable to the Enemy, traffic jams and helps it move faster. Fuel, food, ammunition and other supplies must be made available, which means quartermaster units need to be told where to deliver said supplies and what supplies to deliver. Military Police units need to be told where to direct traffic, and what traffic has the highest priority. Supply and repair companies need to be prepared and ready. Attached air units need to be told when and where to rendezvous with the advancing units if appropriate. Field kitchens need to be set up and provided with food to feed the soldiers as they pass through.
If the unit is in combat, disengagement and relief plans have to be made. A really big operation, such as Operation Cobra, involves moving around enough people and vehicles to handle a fair-sized city. To create the breakthrough from Normandy, Operation Cobra involved moving two large armored divisions (six tank battalions instead of the normal three), four infantry divisions, additional attached artillery, tank, tank destroyer battalions, plus two tactical air wings of P-47 Thunderbolts and most the strategic bombers in the Eighth Air Force for the opening attack. Additional divisions were expected to make holding attacks, attacks designed to both disguise the true objective of the offensive, and to keep the enemy from rushing reinforcements to the most threatened sectors. Giving that many people mission orders is the reason generals have a staff. Moving armies isn't like moving chessmen around on a board. Moving one division can be a really big thing, capable of snarling miles of road. Each and every subunit down to the company level has a defined role in that operation, and thus requires orders that may necessarily involve considerable detail. While commanders like to give subunit commanders considerable latitude in doing their jobs (Division staffs tend to generate the orders for their own subunits, etc) the fact is the order list for large operations can go on for many pages.
Which gets us back to the frag. What each battalion or company staff actually sees is just their tiny piece of this very large puzzle. Soldiers very often don't know anything besides what they themselves are doing. A platoon commander doesn't really need to know what units many miles away are doing. He and his men are cogs in a very large machine. All he needs to know is what his team is supposed to be doing, what is expected of the Enemy, what support he can expect and any friendlies who might be in the vicinity. He doesn't need the full order, in fact it would be bad if he had it and were captured. He just needs his fragment of the larger order making the the army go.
He just needs his frag. Probably that's all he'll get.
haqiqat tells me the current slang is "fraggos", which would differentiate it from an alternative use of frag.