Obviously means Bullshit Buddy, or a friend who is not a good friend but a moocher who forget you exist until they see something that they want from you.
Todd: Hold on man my phone's ringing
Jason: Who is it?
Todd: oh it just one of my BS buddies
Jason: Who?
Todd: Christan, i wonder what he wants now haven't talked to him in weeks, he's not gettin my stuff, damn moocher.
Like fuck buddies or friends with benefits, except your relationship almost solely exists via sexual texts or emails. In fact, you may have only met in person once (or never).
When Sally met Harry at the bar and gave him her number, she hoped he'd ask her out on a date. They texted back and forth a few times, but Harry lived in another state and eventually they became regular sext buddies instead.