Full: For 5...10...11 seconds, while wielding a shield, the next attack skill used against you is blocked. If it was a melee skill, your attacker is knocked down and that skill is disabled for an additional 15 seconds.
Concise: (5...10...11 seconds.) You block the next attack skill. Causes knock-down and +15 second recharge if it was a melee skill. No effect unless you are wielding a shield.
Although they are considered attack skills by other game mechanics, pet attacks will not trigger Shield Bash.
Unlike axe/sword/etc. attack skills, which are disabled if you aren't wielding the appropriate weapon, this skill will not be disabled if you are not wielding a shield. You can still activate it, but its effect will not trigger unless you equip a shield.