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Silly or daft, but according to some it was an expleitive said in a way to avoiding blasphemy thinking the word giddy to be from gidi from gudo or God...
So its supposed to be an alternative for OMG..
That said Id much rather hear oh my giddy aunt or oh fudge, than what we are normally treated to when out and about..
It used to be said to me by Grandma if she thought I was acting the fool. I suppose in UK you might say 'taking the mickey' but in Eire the 'mickey' is something else entirely!