This expression literally translates as "drink blood". Like mosquitoes or vampires do. In this expression, however, it rather means "to tire someone out".
For example, if your child asks you 100 times for the same thing, like doing a chore or buying a toy. You can answer with : "Aret bwar disan do!" Translation: "Stop tiring me or annoying me".
You can also use "bwar disan" at work, especially to describe someone who is lazy. For example, you can say in a conversation with a colleague: "X pe bwar disan" which translates as "X isn't doing their work or X is lazy".
