In the sense of 'I can't believe it!', the origin is uncertain, but it is probably just a euphemistic version of something more profane much as 'Cor blimey!' is a more acceptable version of 'God blind me! One meaning of 'to jigger' was to make a series of rapid, jerky movements such as a fish might make whilst trying to escape a hook. As a result, it came to mean exhausted, tired out or 'done for'. There may just possibly be some connection between the two meanings..."Well, I'm done for!"
in some parts of the UK "a jigger" is a small back alley way or narrow streets behind terraced houses,so it could be s reference to the fact if you didn't know where you were going you would get lost or jiggered ,don't know if that is the correct answer though just a theory
I'm from ooop North, and I've never heard of a jigger?? Gennels and ginnels, yes, but not a jigger. I'm not disputing the fact, but just wondered if anyone did use it, and if so, where? :)