Used as a euphemism for Sex. Sexy sexy sex. Particularly between Arthur and Merlin. Term is used primarily as an excuse when others catch Merlin and Arthur in the act. Rarely works as everyone knows exactly what "poetry" Merlin is teaching Arthur.
Gwaine : Hey Leon, have you seen Merlin?
Leon : Yes. He was with Arthur. In the corridors. Alone.
Gwaine : What was their excuse this time?
Leon : Poetry.
Gwaine : They aren't fooling anyone.