A lackey is someone who works for someone else and tries to get ahead by kissing up to his superiors. For example, a lackey might carry his employer's luggage or fetch her cappuccinos.
A lackey can also be a servant who wears a uniform, like a butler, doorman, or valet. Only the richest, grandest, snobbiest families employ lackeys these days. Another name for a lackey is a manservant, who works in a private home serving the needs of his employer — like a maid, but male. From this earliest meaning came the sense of lackey as a "toady" or "sycophant," someone who fawns and flatters in order to get what they want. The word stems from the Middle French laquais, "foot soldier" or "servant."