"I haven't said anything about it being a phrase in common usage

So what's this then?

Michael, why can't you just accept that in the late 18thc the phrase 'foggy dew' was used to describe the dew that lies on the grass during foggy conditions

A phrase claimed to have been used at a particular, specified time ('late 18thc') surely carries the implication of its having been in common use at that time. How else is one to interpret your "why can't just accept" question above?

~M~