It has legs

There is a reasonably common phrase, "[something] has legs." I'm wondering if it means, figuratively, that something is strong, agile, or has endurance.

Does anyone know for sure which is meant? And, out of curiosity, what's the derivation.

Yes, you got it--strong or has endurance.
It is usually said about a film or play. Many times a film will open with strong ticket sales and if the ticket sales stay strong then the film is said to "have legs"-meaning the inherent goodness of the film can carry it instead of just a flash of curiosity.