"THUNDER THIGHS" are a fat girls thighs that are so humoungous, big & fat that they clap together when they walk. This produces a loud noise which goes like "SLAP, SLAP, SLAP" , hence the "thunder" part of the word.
The thighs of a curvy voluptuous woman, famous in the 70s and early 80s period which adds a seductive, sensual appeal to the whole persona of the woman; unlike a skinny, lean pair of legs so much in fashion and so desired now.
Wow! Check out the thunderthighs on that woman, makes me want to make sweet love to her all night long.