Green’s Dictionary of Slang

mentler n.

a crazy person.

1997		C. McPherson Weir 73: I’m like some fucking mentler, I do be watching it!
2001		C. McPherson Port Authority 10: There was no point in buying stuff for a bunch of mentlers who were going to come and just basically drink you out of house and home.

In phrases

do a mentler (v.)

to act outrageously.

2004

T. Kennedy In Mulligans 40: ‘I did a mentler,’ he says.