By the 1940's the slang term "frog" had crept into the language and was in popular use. It appears in the title of a 1941 patent application for "Frog or Flower Holder for Bowls" and refers to "...adjustable holders for cut flowers of the type commonly known as frogs...".
Catalogs of the 20th century usually refer to the glass, pottery or metal objects made with holes in them for holding flower stems as disks, blocks or inserts. The term "frog" may have originally been used to describe the myriad of small round metal flower arrangers being produced in California from the 1930's through the 1950's
(source book- Bonnie Bull, Flower Frogs For Collectors, Schiffer Publishing LTD., Atglen, PA, 2001)