se croire le premier moutardier du pape

Etymology
lit. 'to believe oneself the pope's chief mustard maker'. From a 17th-century legend with no basis in reality that a 14th-century Avignon pope/antipope appointed his nephew as his official mustard maker, said nephew then bragging everywhere about his status.

Verb

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