It stands for something in a secret language that only members can know. It doesn't translate literally but its ok to tell you that BSD's translation is something between, 'Formula-1 Racing Team', 'Penguins are tasty snacks', and 'We have a better sense of humor than Linux.' :-)
Seriously, BSD is an acronym for 'Berkeley Software Distribution', which is the name the Berkeley CSRG (Computer Systems Research Group) chose for their Unix distribution way back when.