When the person you are conversing with stop replying mid discussion. It is an acronym for 'Blood Out Of A Stone, Mate', suggesting that getting a reply off this person can be similar to squeezing blood out of cold, dry stone.
Andrew: I made three thousand last week on the markets
Kirk: Awesome bro, how did you manage that
Andrew: Got lucky with investments, also starting having sex with my neighbor
Kirk: Awesome, how was she
Andrew: Amazing mate, you'll never guess what she did to my anus!
46 minutes later..