The awkward stance that some males take in order to swell their chest in a ridiculous attempt at make non existing muscle seem present. Generally, the arms are lifted away from the body as if huge lat muscles were pushing them away. The author thinks that looking wide is the same as being wide and believes women will not see the difference.
Dave: Look at that skinny dude over there, he has no muscles but he is swelling himself up like a peacock!
Steve: Yeah he's got invisible lat syndrome, or ILS, common among douchbags who are too lazy to workout for their muscles!