Many civilians as well as military personnel are familiar with the term 4-F (also called 4F). 4-F is a classification given to a new U.S. military registrant indicating that he or she is “not acceptable for service in the Armed Forces” due to medical, dental, or other reasons.
History of the term 4F or 4-F
The term 4F or 4-F started in the Civil War. As both Confederate and Union soldiers were being recruited, there were very few medical or dental reasons for rejection. Quite simply, the battery of tests and screening tools available today simply did not exist back then.
It was noted, however, that in order to properly load a rifle quickly, the gun powder cartridge needed to be ripped open with the teeth. Molars and premolars in the back of the mouth were not sufficient for this task. Only the incisors and canine teeth in the front could be utilized.
Back then, routine dental care did not exist, and many people in their late teens and 20s were missing several teeth. If a recruit could not open the gun powder cartridge with his teeth, he would not be able to reload quickly, placing himself and his fellow soldiers at greater risk.
So, while evaluating new registrants, a dental exam was performed to see if each young man had at least four front teeth. The dentist would examine the young man and evaluate the front teeth (or lack thereof). Those young men without four front teeth were disqualified and not permitted to enlist.
Naturally, a “code” was needed to designate why the registrant was unfit for service. So someone (presumably a Union Officer) came up with: