The name Lou is a girl's name of French origin meaning "renowned warrior". Lou and is often added to lists like Middle Names for Girls and discussed in our forums with posts like "Heaven / Hell / Purgatory Girls".
From the experts:
Lou is usually a short form of Louise, Louisa, or Lucy in English-speaking countries, when Lou is used for girls at all. But in France and Germany, it's a fashionable choice all on its own, sure to gain even more widespread style credibility since Heidi Klum and Seal chose it for their daughter.
At one time Lou was pervasive as a girl's middle name (Mary Lou, Betty Lou), and seems to be making something of a comeback--Louis C.K. has a Mary Lou and Keri Russell named her daughter Willa Lou.