LB simply stands for "Loose Butthole". This can be used to describe yourself or other people, and it's connotation depends on the situation at hand. Its meaning can be equated to "getting anally penetrated" or just being a sexually loose person. An alternate meaning could be used in the context of being recently bested by a person or task.
John: "Hey have you taken the chemistry test yet?"
Bart: "Oh my god its so hard. I'm so LB right now."
John: "Dude what do you know about Jessica?"
Bart: "I heard she's super LB man go for it."