Person who holds views typical of the Daily Mail and/or its readers.
Generally assumed to be a rabid right-wing racist who holds everyone (except people like them) in the highest suspicion; to be self-centred and think that everyone should like what they like and do what makes them happy. In essence, be like them.
"I'm worried about Henry, he keeps on criticising the NHS and mouthing off about Islam."
"Watch out for him, sounds like he's turning into a Mailtard."