Definition 1 of 1
Such a reference is a warning for you to be extra cautious and advises you to behave and speak with that Mental guy in a somewhat non-kindling way.
You will find at least one gangster nicknamed " Mental XYZ" in most cities.
( Hey sonny Konga, be careful. That Mental Maada is a bit odd these days. For some days avoid provoking him.)
Note: Kannada gang slang has Konga for a Tamilian, Kindlu for kindle 'rong' for wrong which is supposed to actually mean 'odd'.