Describes a feeling of wobbliness or unsteadiness. You can barely stand, and your knees may completely give out at any second. The sensation may well be a precursor to fainting and accompanied by a shortness of breath or quickened heart rate.
There are many, varied causes. A very common one is fear. Also possible are extreme excitement, surprise, and happiness. An attractive woman raising her eyebrows, biting her lip, and looking up at you over her glasses is perhaps the most desirable of reasons.
Should you feel weak in the knees, the best course of action is to identify the source of your distress and move as quickly as possible. Whether you move towards or away from the source is entirely a personal decision.