A Menc is a person who has lost touch with reality in some way; who embodies self-delusion as their illness of choice and who affects you to the point of incredulity, regarding their detachment from what is obvious to everyone else.
Politically incorrect to associate with disabled people; rather being reserved for merchants who hide their malingering ways behind a diagnosis of mental illness and who drain people they come into contact with, with their predictable antics.
Specifically, somebody who expects you to believe their serial blags, such as "Sorry I missed my appointment today, dodgy curry last night" to which one's automatic internal response is "tiresome Menc".
Menc is a term used to directly define a person who has a mental disability such as downs syndrome or is mentally inept in some way. It can be used in a jovial sense, for example toward a perfectly normal person who is acting strangely, carelessly or just fooling around. Possible origins of the word Menc include Mencap, the UK based charity which promotes Mencs in an attempt to integrate them into society, whether we like it or not.
Tim, why can't you follow a simple instruction? You're such a menc.