stands for Negligent discharge
A negligent discharge (ND) is a discharge of a firearm involving culpable carelessness. In judicial and military technical terms, a negligent discharge is a chargeable offence. A number of armed forces automatically consider any accidental discharge to be negligent discharge, under the assumption that a trained
soldier has control of his weapon at all times. This is the case notably in the
United States Army, Canadian Army, the Royal
Air Force and the British Army.