Refers to the group of investors who hold a short position and are quick to exit their positions at the first sign of strength in the underlying asset. This group of investors looks to capture the gain on a move lower, but they are usually unwilling to take on as much risk as other investors.
Weak shorts differ from other traders because they will close their position at the first sign that it will move against them. It is not uncommon for this group of investors exit a position only to see the asset move to a price that would have made the trade profitable if they had left it open.