When a guy,has very strong thighs, which makes them powerful. Hence Power thighs.
Girl 1:Wow that guy over there has Power Thighs.
Guy: I wish I had thighs like that.
Thighs that are supremely powerful, that are sculpted by angels. You can usually tell through somebodies trousers that they have power thighs.
Adam: Hey alex see that guy over there, he totally has power thighs.
Alex: nahh, they are nothing compared to mine.