Il est constamment perdu dans ses rêveries, il n’a pas les pieds sur terre.
He is constantly lost in his daydreams; he doesn’t have his feet on the ground.

Elle a des idées fantastiques, mais elle ne semble pas avoir les pieds sur terre pour les concrétiser.
She has fantastic ideas, but she doesn’t seem to have her feet on the ground to bring them to life.

Malgré son succès, il reste humble et n’a pas les pieds sur terre.
Despite his success, he remains humble and doesn’t have his feet on the ground.

Elle vit dans un monde imaginaire, elle ne semble pas avoir les pieds sur terre.
She lives in an imaginary world; she doesn’t seem to have her feet on the ground.

Il est tellement distrait qu’il semble ne pas avoir les pieds sur terre la plupart du temps.
He is so absent-minded that he seems to not have his feet on the ground most of the time.