If you are incapacitated, you can’t do what you normally do, what you’re being asked to do — or perhaps, much of anything. To incapacitate someone is to cause him or her to be unable to function normally, like a bad cold that incapacitates you.
The verb incapacitate is related to the word capacity. Capacity comes from the Latin word capacitas, meaning “that can contain,” or how much something — brain, bucket or otherwise — can hold. The prefix in- reverses the meaning and when the suffix -ate is added, incapacitate means someone has been made unable to “contain” much — like attention, effort, or energy.