Basically, you have normal thighs, then they get fat (tier 1). But when they get like even fatter, you get another level of fat like a hill of fat(tier 2), which is loose off the other fat so when a thunder thigh is being walked besides the legs (tier 1) rubbing against eachother they (tier 2) hit eachother making a noise, hence thunder coming into the expression, most likely to because of alliteration for a dramatic, comedic affect. This sound can have a brighter tone and richer feel if accentuated with bumpy cellulite
when a chick's booty is so fat her thighs look like canons of muscle that could choke a horse. men are initially attracted because of the hourglass effect but once she gets naked you realize you're really just having sex with an enormous woman.
chick thinks she's hot but truth is no one wants those 50 pound thunder thighs. god damn woman put down the chicken wings and eat a salad.