A condition when a wannabe bodybuilder or muscle man, usually under age 18, think their are lats are bigger than they actually are. The wannabes carry their arms out to the side arched downward in an exagerated fashion pretending they have big muscules. If the lats were truly developed, this would be a normal posture. There is no physiological reason for the arms to stick out if the lats aren't there.
Lats - derived from latissimus dorsi. Back muscules that give males a "V" shape when well developed.
Air - meaning non-existent or invisible.
Air lats to a bodybuilder is what an air guitar is to Jimmy Hendrix.
Air lats bragged that he could bench 350lbs.