It usually means that you are zoning out or "not there" by not focusing on the world around you.
Guy: "I swear you seem like your stuck in the clouds everytime we are on a date."
Girl: "That is because Im thinking about what we are going to do tonight."
Guy: "Oh."
You hang upside down using your legs on a bar while your partner has their dick ready to be sucked. You grab his waist to force his dick into your upside down mouth. Staying upside down for long isn't a good idea so make sure you take breaks every time after he cums. This probably improves leg strength?
Bob: Oh hey dude my GF just gave me a Monkey's Blowjob!
Walter: Damn her legs must be pretty strong
Bob: Hell yeah they are
Walter: Thigh guy.