se croire sorti de la cuisse de Jupiter

Etymology
lit. 'to think oneself came out of Jupiter's thigh' (like the goddess Athena/Minerva and the god Dionysus).

Verb

 * 1)  to think one is God's gift to the world, to think one is the business, to be full of oneself, to think one is the bee's knees, to think highly of oneself