En mi opinión, la traducción de phat depende mucho del contexto. Por ejemplo, si se refiere a un carro... that car is phat, se podría decir tremendo carro. Si dicen que un rapero es "phat" yo diría tremendo rapero.

Fuera de serie es otra opción, dependiendo del contexto.

Hay muchos hispanohablantes que usan la palabra cool, así que cool o super cool (para darle énfasis) también sirven.

Si se refiere a una person entonces quiere decir que la persona es sexy o super atractiva. Por ejemplo She's phat sería ella está buenísima o es bien sexy...

phat (adjective)
/fQt/
Definition: an adjective meaning 'pretty hot and tempting'
Attested: I first heard this used in a study lounge by a girl from Staton Island, New York, New York
Example: "I can't believe that you got into the party without paying. That's so phat."
Etymology: At one point, the saying 'pretty hot and tempting' was probably very popular. This phrase being excessively long, it was probably easier to form an acronym that reatained the adjective phrase quality of the original statement. The word seems to have originated near cities, but then to have spread to many places, including Cornell. Many people are aware of the word phat, and how to use. Most, however, do not know what it stands for.
- Ryan Rapp
http://instruct1.cit.cornell.edu/Courses/ling100_4/DLU-Hydre...

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Note added at 8 hrs (2006-04-18 20:00:29 GMT)
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lo máximo es otra opción para phat...

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Note added at 1 day8 hrs (2006-04-19 20:03:05 GMT)
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adjective
phat

Rich like butter, really good, extremely well put together. Synonyms: cool, dope, fly. "ATCQ be putting together some phat beats" -- A Tribe Called Quest
Incorrectly attributed to mean "PHysically ATtractive," "Pussy, Hips And Tits," or "Pretty Hot And Tempting." See [1]
Phatfarm, the clothing and sneaker line
http://www.rapdict.org/Phat


Phat


Nice, beautiful, plentiful.
http://www.voxcommunications.com/slang15.htm

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Note added at 1 day8 hrs (2006-04-19 20:06:08 GMT)
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phat
1. (adj) Cool; very good; very attractive. Man, she is phat. [Georgia Tech, Atlanta, GA, 1998; North Carolina Central University, Durham, NC, 1998; Clark Atlanta University, Atlanta, GA; Jefferson County, MO] 2. (adj) Pretty hot and tempting. Your girl is phat! [Central Oregon Community College, Bend, OR, 1998; University of Nebraska, Lincoln, Georgia Tech, Atlanta, GA] 3. (n) Acronym for: Pussy, Hips, Ass, Titties. Man, she's got phat. [University of Wisconsin-Madison, Madison, WI, 1998]


http://www.csupomona.edu/~jasanders/slang/csrpp.htm

Phat has all of these uses and definitions, and I don't think there is one term in Spanish that captures all of them.