Someone who "throws like a girl" probably just has bad footwork.

As Lord Brawl noted previously in this node, in the act of throwing an object, one generates power from the shoulder, abdominal and leg muscles; one uses the elbow and the wrist for accuracy.

This is all understandably counter-intuitive for a novice; throwing seems like something one does with the arm, but if you don't use the rest of your body, your throw will have little distance.

In a proper throwing motion, one should step forward with the opposite foot just after cocking the arm. While doing this, the thrower should twist his/her torso a bit toward the throwing arm. Then, in the process of throwing the object, the person will naturally transfer his/her weight to the forward (opposite) foot during the forward throwing motion and twist his/her body from right to left (for a right-hander) during the follow-through. All this will add power.

Unfortunately, many people who haven't been taught how to throw properly try copying their peers and end up stepping forward with the incorrect foot — i.e., the right foot for a right-handed person. This will definitely make you throw poorly and will look like a total dork as well. I'd guess this is the real genesis of the "throws like a girl" insult.