Why does “trip the light fantastic” mean “to dance”?

Question

This is a really cool phrase. It certainly evokes imagery of dancing about nimbly on a ray of light, or something of the sort. But how does it make sense? "to trip" I can see as being kind of like dancing. "to trip the light" - which light? And what part of speech is "fantastic" here?

Answer 1

The common reference (as mentioned for instance in wordreference is from a verse in Milton’s L’Allegro (a pastoral poem by John Milton published in 1645).
The Wikipedia article list all the other references/origins.

"Come, and trip it as you go, on the light fantastic toe."

This article illustrates the result when it is presented as modern ballet:

L’Allegro, il Penseroso ed il Moderato is uncategorisable - it’s both dance and oratorio, both narratively coloured and yet at times more formally complex than purest ballet.

Milton's phrase, 'Trip it as you go/ On the light fantastic toe', could have been minted for Mark Morris.

The Stage and culturevulture concur:

There is a verse in Milton’s L’Allegro that may have inspired Morris to craft this highly innovative dance: "Come, and trip it as you go, on the light fantastic toe."
His dancers can trip the light fantastic with the best of them [...].