Using a spear directly against a shield is generally inadvisable. The shield is probably made of something dense and rather sturdy. Hitting it head-on with a pointy object is likely to blunt the point. A spear without a point is called a club or a quarterstaff, depending on whether or not you know anything about quarterstaff fighting. Either way, you were better off before driving the spear into the shield.

Even if your spear remains sharp, it is now lodged firmly in the shield and will be difficult, if not impossible, to remove.

This is a practical application of a general case. Most edged weapons are not designed to be used directly against a solid, hard material such as plate body armor or a shield; attempting to do so will damage the weapon much more than it will damage the armor. There are exceptions, but most of them are tools as specialized in their way as a can opener is in its way.

Even though using a spear directly against a shield is inadvisable, this proverb has much to be said for it. Testing the claims of blustering merchants, and of others who try to make extravagant claims, is a worthy and necessary part of life.