What a member of the armed forces goes on, or becomes when they no longer feel the need for physical training. This is usually typical of a substandard individual, or 'turd' as they may be called from time to time. At times, some of these 'light duties' have a legitimate reason for not participating in physical training, however, most light duties will make themselves known at the most opportune time, ie, before a physical fitness test, or Company run. They usually will claim an ailment that is hard to diagnose, ie, back pain, knee pain, and the ultimate.. shin splints.
Guy #1: Man I really don't wanna run tomorrow. My favorite singer Adam Lambert is gonna be singing on Logo, and running will interfere with my duties as the Elton John Fan Club President.
Guy #2: No problem! Just go on Light Duty! That way, we can watch Adam Lambert all night long! Wanna make out?