Also Imaginary Lat Syndrome, an affliction of certain bodybuilders and bodybuilder wannabes in which the sufferer walks around with his arms out by his sides like he has a suitcase tucked under each armpit.
The latissimus dorsi are thick, winglike muscles running down the side of the back, and in a sufficiently trained bodybuilder can be flared like a cobra's hood and visible from the front. It is an important component of the bodybuilder look to have large lats, in that it makes the shoulders appear massive and tapering down to the waist makes the waist look smaller. It's also an impressive as hell display, making the body look more massive.
Because bodybuilders need to constantly be flexing in order to impress the judges subconsciously in competition, they tend to affect a stulted, "rigor mortis" type walk with the arms out to the sides slightly, allowing room for the lat flare to be seen. This has been mocked by the Dilbert cartoon in which Dilbert does this after one gym session, telling Dogbert it must be "some kind of armpit muscle".
It also means that the do-rag wearing guy with the Monster Energy logo on his spinning rims and the stringer wifebeater T-shirt who hangs out at the gym on a Friday night to "get a pump" before heading out to the bar - all bicep and chest, no legs - often affects this same strange walk even though he's nowhere near a competition stage and has no valid reason to be walking like that. An example here. Everything about this video is subject to scorn - from the board shorts to hide a lack of leg development and the fact that he's making a huge to-do about bench pressing a mere 135lb. And ILS is literally the perfect icing on this gym-fuckery* cake.
When that kind of person does it, the pejorative term is "ILS" or "Imaginary Lat Syndrome". It is ALWAYS pejorative, and labels the person doing it as a poser and a wannabe.
*Facebook page, "Gym Fuckery". Catalogs all kinds of random gym weirdness.