The morning star. Venus is both an evening and a morning star: When she follows the sun, and is an evening star, she is called Hesperus; when she precedes the sun, and appears before sunrise, she is called Lucifer (the light-bringer).
Proud as Lucifer. Very haughty and overbearing. Lucifer is the name given by Isaiah to Nebuchadnezzar, the proud but ruined king of Babylon: “Take up this proverb against the King of Babylon, and say, ... How art thou fallen, from heaven, O Lucifer, son of the morning!” (Isa. XIV. 4, 12). The poets feign that Satan, before he was driven out of heaven for his pride, was called Lucifer. Milton, in his Paradise Lost, gives this name to the demon of “Sinful Pride.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894