Cow over the moon.

I recently came across this phrase - 'And the cow jumped over the moon'. What does this mean?

If I were to use it - which I never have - I'd use it to express disbelief,

It's part of an old nursery rhyme -
'Hey Diddle, Diddle, the cat and the Fiddle, the Cow jumped over the Moon. The little Dog laughed to see such fun, and the Cat ran away with the spoon'

The dish ran away with the spoon, as I remember it. I also vaguely remember hearing an explanation, although I cannot recall where.

psi

Hey Diddle Diddle
By Randolph Caldecott

www.randolphcaldecott.org.uk/ rhymes.htm

I apologize:
Randolph did the illustrations (not the writing)of many childrens books.
Please see www.rooneydesign.com for info on Hey diddle diddle.