1.Verbthird-person singular simple present indicative of lean
2.NounA dangerous sensory illusion where the pilot of an aircraft believes themselves to be in a bank when they are actually wings-level, or vice-versa, often resulting in spatial disorientation and loss of control of the aircraft.
"If you don't use your flight instruments when flying in cloud, you can easily get the leans and enter a fatal graveyard spiral."