Community wiki because it is answered over at <A HREF="https://math.stackexchange.com/questions/674961/how-to-pronounce-the-partition-relation">MSE</A>. 

<IMG SRC="https://ilorentz.org/beenakker/MO/arrownotation.jpg"/>

<A HREF="https://books.google.nl/books?id=tCATRdtV_cwC">source</A>