Climbing slang. References the build up of lactic acid in the forearms.
When climbing steep ground the forearms become distended and the skin tight, with main veins very visible after large amounts of excertion, literally pumped up.
Also seen as pumpy, "That route is way pumpy (dude)" - The route requires a lot of brute strength to ascend. Note that a pumpy route is not automatically technically difficult.
Or pumped, "I couldn't do the crux as i was too pumped" - The climber had run out of strength and was unable to complete the climb. This is also a well worn excuse for not being technically skilled enough to do the climb or too scared to complete the climb.