Your boo, your girlfriend/boyfriend. Derived from the tagalog word for flower (bulaklak) it is used as a term of endearment but also can be used in lewd terms.
"The boo lack lack has me on lock down, I can't go to the strip club."
In lewd terms:
"Ay gurl, let me get all up that Boo lack lack."
"So I walk into the restroom and I hear these 2 people boo lack lackin in the stall."