Nowadays the word means that a person is attractive. Fit as fuck = a very attractive person.
Women aspire to be thin, as thin as possible, as this is what they think fit is.
THIS IS WHAT FIT LOOKS LIKE:
This woman is a trained athlete, a world class champion, and now an Olympic gold medalist!!!
THIS is the body girls should be aspiring to; a healthy, balanced and FIT body.
Who wants to look like this:
When you can look like THIS: