Slang. Someone who is a 'mental' and subsequently talks to themselves.
Derived from people that speak on hands free and look like they are talking to themselves (as there is no one visibly present) but are in fact having a phone conversation.
The 'mentals' on the other hand, have the exact same conversation... but they ain't plugged in to no one!
Man that Penelope is hands free. I was on the bus the other day and she was there chewing her hand and talking to herself.
Poor old Billy ever since he caught his mum engaged in bukkake with the local football team and his dad then committed suicide and his pet dog got run over and his house got burnt down and he lost his arm in a chainsaw accident, has gone a bit handsfree. Man, I swear down, I saw him in the street the other day, shouting at himself.
the incredible ability to masturbate without any contact with one's genitals. a skill the male japanese business class has cultivated for decades.