تعریف و معنای "before {sb} (can|could) say Jack Robinson"

What is the origin of the idiom "before someone can say Jack Robinson" and when to use it?

The exact origin of the idiom "before someone can say Jack Robinson" is uncertain, but it has been in use since at least the 18th century. It is believed to have emerged in England and is often employed in storytelling, anecdotes, or casual conversations to describe instances where something happens rapidly or unexpectedly.