Learn English – Why “job” in “nut job”

The Wikipedia entry has sourced its answer from the Oxford English Dictionary entry for 'mackem'. As far as word origins are considered, probably as authentic as it gets.

EDIT: As it seems that everyone cannot access the OED link, here's an excerpt. There are two possible origins attributed:

Probably with allusion to the phrase mack 'em and tack 'em and variations thereof, freq. said to refer to the shipbuilding industry of the region

or..

Perhaps partly also with allusion to the pronunciation of make typical of Wearside, as contrasted with that of Tyneside.

The word indeed comes from "head shrinker", and likely originates from the "shrunken heads" of tribal rituals. One possibility from World Wide Words:

All the early evidence suggests that the person who invented the psychiatrist sense worked in the movies (no jokes please). We have to assume that the term came about because people regarded the process of psychiatry as being like head-shrinking because it reduced the size of the swollen egos so common in show-business. Or perhaps they were suspicious about what psychiatrists actually did to their heads and how they did it and so made a joke to relieve the tension.