Hot dog legs... Well it's another one of those viral internet things that will probably die out before I even finish writing this answer..
Anyway it's basically the phrase used to describe the kind of photo that a girl takes showing off her legs while sitting at the beach or beside the pool.
The legs are meant to look like a pair of hot dogs.
Sounds silly, but there's a whole Tumblr blog about it. And check out the pics below, which ones do you think are hot dogs and which ones are hot legs??