Some creatures are clumsy while using a specific movement mode (noted next to that mode in the creature’s stat block), and others are clumsy while on the ground (noted next to the creature’s speed). While a creature is clumsy, it takes a -4 penalty to attack rolls and all defenses.

Published in Monster Manual 2, page(s) 216, Rules Compendium, page(s) 309, Monster Vault: Threats to the Nentir Vale, page(s) 122.