This is a common phrase in the United States. It refers to someone doing worse than they have ever done. This usually refers to someone who is somewhat amoral going out and doing something even more lowdown and dirty than they normally do, but it can refer to any substandard effort.

Often stated in the past sense as "You just sunk to a new low." or something similar. It is usually said at least in part to shame the object into putting forth a better effort or repenting of whatever actions brought forth the comment.