“Bone Crack” is an ability that a thief can acquire through stealing from certain mobs. Its tooltip says “Stun your foe”.
It also indicates the duration of the stun. However, there are two durations listed, one says 3 seconds and the other says 2 seconds.
Unfortunately I can’t post a screenshot because if I take a screenshot while mousing over the icon so that the tooltip shows, the screenshot doesn’t have the tooltip in it.