Andrew C.Oliver writes about XHSSF which according to this could be translated to eXtremely Horrible SpreadSheet Format.

How about putting my HMM (Horrible Marketing Machine) to work on this one? read on...

Building on the recognized nightmarish features of our industry standard HSSF format, OurCoolCompany is proud to introduce the next step in HEI (Horrible Enterprise Integration) - the XHSSF format, bringing HSSF to a new level.
With XHSSF, the Horrible structures of HSSF can be accessed in XML form using common tools, lowering HEI costs dramatically by allowing HEI applications to exchange Horrible Structures and the corresponding Implementation Nightmares (IN) directly, allowing INs to spread faster through HEI networks.
XHSSF licences are available for a Horrible Nominal Price (HNP) to existing HSSF licensees
Please contact our Horrible Sales Department (HSD) for more info.

Sorry Andy, can't resist ;-)