That's what "Rap Dictionary" offers:

PHAT

1. Rich like butter, really good, extremely well put together. "ATCQ be putting together some phat beats."

2. If a girl was phat, she had Plenty o' Hips And Thighs, in other words PHysically ATtractive; old slang. Think 60's or early 70's here. A bit more modern: Pussy, Hips And Tits or even Pretty Hot And Tempting.

3. Another word for cool. "Yo dat show's phat"

4. Phatfarm the clothing, sneaker line.