The term gadzooks (where gad was used to refer to god) has a religious origin.  It is a mild or indirect way of referring to the phrase "God's hooks", which is a reference  to the nails used at the Crucifixion.  The word was first used in the 1600s. 

 After the term gadzooks fell from regular use, the term gadzookery was adopted (sometime around 1955) to refer to the deliberate use of old-fashioned language. 

4 hours ago, Valkyrie said:

I feel like Scooby Doo needs to be mentioned here. 

@Valkyrie

Gadzooks!!! I agree, but I was gobsmacked, that you felt that way too!