Crikey is a mild word you blurt out when you are surprised, or a bit annoyed — a soft stand-in for a stronger swear. “Crikey, that’s expensive!” “Crikey, I didn’t see that coming.” It is very British and Australian, a little old-fashioned and a little comic. Younger people still use it, often with a smile. Steve Irwin made it famous around the world, so some people outside Britain say it as a joke. It started in the 1800s as a polite dodge for the name Christ (perhaps from “Christ King”), so you could sound shocked without using a holy name. Cripes, criminy, and Scottish crivvens are the same kind of polite shock-word. It is speech, not writing; you would not put it in a school essay. It is never a noun or a verb — just the little explosion at the start of a sentence.