Backfire burn
No spellchecker was used on this shirt before it went to the printer, and it appears no one bothered to check for grammatical errors. We believe that she is warning you about your boyfriend and advising you to watch your back before he finds someone else.
That may have been a barely amusing phrase, but this t-shirt has now made the wearer the laughing stock. Unfortunately, when it comes to stealing anything, the grammatical errors don’t assist her case.