Usually applied in a military context but can be used in general social contexts.
alfred: Dude just have another couple of drinks.
winston: Sorry alfred, i gotta lan, im going BTB.
The other meaning for BTB is an attribution to ones extreme lies- pyschodynamically an impulsive need to lie. Related to the former meaning through the use of bollock, which here will mean a load of rubbish or a lie instead of testicle.
Hey, thats a load of BTB and you know it is. You dont play Rugby for, Wales, England, Fiji and Germany Fuckface!