An individual who is not currently fat, but likely will be in the future, particularly after they age, bear (a) child(ren) or stop working out 3 times a day and start eating because they get married and thus tie their man down.
Often prefat is used as a derogatory comment, and with consideration not just of one's physical appearance but also their personality.
That whore at work may look decent now but she's prefat. Giver her 3 years, will be round as a ball.
Christy better be careful; I heard her say she want's a big family but you know she prefat. Those pounds aren't coming off after she pops a kid.