Pumped is a term used to describe the tight, blood-congested feeling in a muscle after it has been intensely trained.  This tends to occur in the forearm, especially in new climbers, or after a particular strenuous climb.  “The pump” may be a body builders dream, but it’s a climber’s nightmare.  One a climber has pumped forearms, their climbing session will be close to being over. 

One point wroth noting, especially if new to climbing though, is that pumped forearms are often caused by the climber gripping too hard on the climbing holds.  In time when nerves and technique improve, it will take longer for forearms to be pumped.  Like everything, practice and training will improve this, so don’t despair when it happens to you.