Phat isn’t a shortening of a salacious phrase applying to the physical attributes of women; it is instead a deliberate
misspelling of fat, a word that has for centuries carried in the
English language (and some others) a meaning of “rich, abundant, or desirable.” Phat was accepted purely on its own merits, in the same manner that "boss" or "cool" are
understood as general terms of approval without anyone’s attempting to justify their existences as acronyms for “bitchin’ overpriced success stuff” or “crazy outlook on life.”
Phat’s earliest print sighting dates to a Time magazine entry of 2 August 1963 identifying it as “Negro argot,” with “mellow, phat, stone, and boss” all being “general adjectives of approval.”