Christian Siriano wrote a book about "how to be fierce" a few years ago; I think the term is starting to be a little less trendy than it used to be. Refers to those who have strong self image and are fearless in their personal style, life, career etc.: the total package, people who have it all, do it all, look the part, are intelligent and really care....
Nah it's mostly just a pyschobabble phrase that caught on and used to sound cool but now sounds slightly dated. Expired, as Christian would say.
http://www.youtube.com/watch?v=_kI_vFYQ_lc