The 36-year-old's actions will see McGinn ruled out for three weeks with injury.
According to the Daily Record, the incident arose after McGinn, 20, robbed Thompson of the ball in training. The latter picked up a spiked pole and threw it McGinn's way. It buried itself into his thigh, and the cut was bleeding profusely.
Thompson, as reported by the Scottish Sun, said:
I've been shocked by it, to be honest, really down about it. I like a laugh and a joke in training, but for something like this to happen is just unbelievable.