Space Walk is a condition that occurs to UNSC personnel that spend too much time in microgravity. It is likely that their legs become unaccustomed to holding their body weight due to muscle mass lost during extended periods of free floating caused by zero gravity. As a result, when they return to an atmosphere with stable gravity they have a strange lope to their walk. This is called "space walk" by crewmen. It was first mentioned in Halo: The Fall of Reach when introducing Franklin Mendez.
Note, however, this term is only a nickname for Muscular Atrophy, an anatomical pathology.