A vivid word from news:

This boo is a slang, derived from the French word "beau" meaning beautiful. However, nowadays people use it for girl or boyfriend, or really close friend.

Here is where I saw it - two days ago Anderson Cooper on CNN discussing O-H "what's next" with bunch of folks, including Donna Brazille. Anderson C situationally said "I want to be your boo", everyone was laughing laud... Watch

[internuts]

Here "girl or boyfriend..." should be "girlfriend or boyfriend..." Don't want to be misleading :-)