Typically a person that tries to throw an object but is so weak in thier arms and upper body that said object fails to make any reasonable distance causing ammusement to everyone witnessing it.
1. I first came across this term in the Australian Army, when the instructors would criticize the females practicing grenade throwing who could barely throw the grenade five meters.
2. The (cricket, baseball etc) coach yelled "muppet arms" at the top of his voice every time a player failed by throwing with pathetic force.