Definition: Someone who flatters people of influence in hopes of having some influence spent in her direction; a person who seeks to further himself by licking the boots of his superiors; a "yes man."
Usage: The rather worn joke goes something like this—Lackey: "Yes, boss, whatever you say, boss." Boss: "'Yes? Yes? Why do you say 'yes' to whatever I say? What kind of sycophant are you?" Lackey: "Um, what kind do you want me to be?" The noun is "sycophancy."