BWBR

A BWBR is a brunette who acts like a "dumb blonde." The "joke" here is that the brunette is dumb, and blondes are dumb, so the brunette's true hair color must be blonde.

This acronym is not only mean-spirited, but also assumes the person receiving it has prior knowledge of (and appreciation for) "dumb blonde" jokes. At this point, the "dumb blonde" stereotype is pretty much played out, so you may want to counsel the person who sent you BWBR to stop making assumptions about people based on their hair color.