Mikiko Lockwood in an article on The Hosta Library titled, A Little About Japanese Hosta Terms defines the term shiroshimafu or shiro shimafu as white-stripe(d) or white streak(ed) and the term iwa as rock or Iwa Gibōshi which is the Japanese name for the species H. longipes.

This cultivar name was mentioned in the Lockwood article with no further details but we can probably expect that it is a plant with white striped or streaked foliage found in a rocky environment and is related to H. longipes.