"phat"

[Jonathon Green]

As well as those already suggested, I have found the supposed ety. of phat attributed to such acronyms as physically attractive, pretty hips, ass, tits, or pretty hot and tempting. There are doubtless others. None of which, I would opine, are pertinent. The word, which dates from the 1960s, although it gained a wider popularization through the 1980s+ hip-hop/rap culture, seems to be a deliberately 'slanged-up' spelling of fat, meaning 'first-rate, excellent', which can be found in such cites as 

1968 'Hy Lit' Hy Lit's Unbelievable Dictionary of Hip Words 14: fat - Great; cool; you dig it; boss; likeable; nice; ultra-hip. 

That said, the 'congratulatory' use of fat can be found as early as the playwright Middleton (c.1610).  

Jonathon Green