No, just checked "The Art and Science of..." by our very own SC and found no mention of the term. We can safely assume that it is either an Australianism for the lack of rigidity on the rotor inplane or a made up phrase by the question setter.
These are not as rare as you might think. The exam writer (who may not be a helicopter pilot or engineer) may have taken the phrase "Rigid inplane" (sic - one word) from a POH (Robinson uses the phrase) and developed his own antonym.
As to the aerodynamic effect of an inplane that flexes or is "soft" the subject is way beyond my level, but there are people here who know and may be able to help.