Demented (adjective)

1. Suffering from severe mental illness, especially dementia.
2. Acting in a crazy or irrational way.

Origin:

From old french dementer, from latin dementare, "to drive out of one"s mind".

Examples:

1. The patient was diagnosed as being demented.
2. The old man's behavior had become increasingly demented.
3. The criminal's actions were those of a demented mind.
4. The patient's dementia caused him to become demented.
5. The patient's behavior was so demented, it was hard to talk to him.