Early Nineties.
Al Gore invented the term "she's hot" in an early attempt to promote his lies on global warming. The first webpage on his newly invented internet was called women-who-look-better-than-tipper-are-bad-for-the-environment.com
On it, the VeePee propagated that every female with charming looks radiates a constant sexual aura of 12,000 Terajoule (in particular: such hotties like Tera Joy and Tera Patrick). Studies from paid scientists at the Tennessee State University proved that such an enormous outburst of hotness would inevitably lead to the eventual extinction of cuddly polar bears, cute penguins and Picabo Street.
While the idea of Tera Patrick killing polar bear cubs was disproven on Mythbusters a week later, the idiom "She's hot!" was adopted by the general public quickly after the Ken Starr report revealed that Bill Clinton's generic excuse to take off his pants was "Oh, it's so warm in the Oval Office. There must be a HAWT women somewhere, radiating like crazy!"
(-> please note that Clinton was the first black president, so of course he used the ebonics term "hawt")