@Vierkantor In Spanish, "estar listo" means "to be prepared/ready", but "ser listo" means "to be smart". (Estar is for states, ser is for characteristics. Something like that.)

@varx The linguistic history of "listo" is also very interesting! "listo" meaning "smart" might have been borrowed from a form of Germanic "listiz" (art, craft; crafty), and then one of its meanings shifted via "quick" to "ready".

This is one possibility though. The counterargument against this idea is that a related word to "listo" appears in many Romance languages with the meaning "ready", and only in Spanish and Catalan it also has the meaning "clever".