I've heard this term somewhere in TF2, and it also goes along the lines of "pocket soldier", or "pocket scout", or something like that.
What does the "pocket" mean?
A pocket medic is a medic that only heals his "pocket", that is one person in specific. The term is typically used pejoratively or neutrally, because the medic is supposed to keep the entire team healthy, not just his friend who's playing Heavy.
In random games, however, pocketing can be effective with friends or people with a clue. You know who you're ubering, you know you can trust them, you know you work well together. Just do make sure you share the heals as you build your uber.
A slightly different use of the word pocket is reserved in 6v6 competitive TF2 in order to distinguish the pocket soldier (who sticks with your medic) and the roaming soldier (who ambushes the enemy medic).