In the context of how I've heard it being used is, having so many outbound links from your site that essentially more
PR is being passed to them than what you would be passing in between your own pages.
Usually t's something that is said to be averted by not having a gazillion OBL's on every page, using "rel="nofollow"", or other methods of supposed
PR 'sculpting'.
The term may be used in other contexts. I'll let others chime in on their views on this one.