In C. S. Lewis's Chronicles of Narnia, I noticed the repetition of the phrase "faster than you could say Jack Robinson" as a descriptive sentence in both The Lion, The Witch, And The Wardrobe and The Magician's Nephew. Was this a phrase of the time, a saying coined by Lewis, or a deliberate reference to an in-universe person?
"Faster than you can say Jack Robinson" (meaning very quickly) is a common English expression, from well before C. S. Lewis's time to the present day. The online Oxford English Dictionary (paywalled but you may have access through your public or university library) has citations ranging from 1763 to 2016. Quoting from the Wikipedia page for Jack Robinson (mythical person):
Jack Robinson is a name present in two common figures of speech. When referring to Jack Robinson, it is used to represent quickness. In contrast, the phrase "(A)round Jack Robinson's barn" has the opposite connotation, implying slowness, as it is often used to refer to circumlocution, circumvention, or doing things in roundabout or unnecessarily complicated ways.
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Multiple citations explain references to Jack Robinson as meaning quickness of thought or deed. The normal usage is, "(something is done) faster than you can say Jack Robinson", or otherwise, "before you can say Jack Robinson". The phrase can be traced back to the eighteenth century.