When, instead of a brain, a person has a third lung in the space where their brain should be. Unfortunatley, this means they have a bigger air capacity, allowing them to talk more, but because they have no brain, they don't have the ability to think through about what they're going to say first.
A common name for somebody with TLS is airhead. Which is ironic because lungs of course breathe in air.
Guy: '*cough* TLS! *cough*'
Sarah: she has TLS.
Katie: YEAH! I HAVE TLS!!!! A LOT!!!
Girl 2: yes, and I was totally thrilled that he suffers from TLS