Being comatose is being in a coma, unconscious and unable to communicate, often for long periods of time. A bad illness or unexpected accident or injury — especially to the head — can make you comatose and trapped inside a body that isn't working.
Comatose comes from the Greek kōma, "deep sleep." When you're in a deep sleep, your body is still and you don't respond to things around you. Being comatose means being in that sleepy, unresponsive state and not being able to get out of it. A much less serious use of this adjective is as a description for getting really tired while doing or watching something, like when you feel comatose after a chemistry lecture — assuming chemistry's not your thing.
