What does mephitic (from Man and Superman) mean?

If anyone ever says that you walk on mephitic clouds, be ready to defend yourself — or be embarrassed. Mephitis is a harmful, bad-smelling vapor from the earth — for example, the odor from decomposing organic matter or poisonous gas from a mine. It can also refer to just a plain ol' stench or bad smell.

In George Bernard Shaw's Man and Superman, Don Juan expounds on romantic illusion:

The great central purpose of breeding the race ... that purpose which is now hidden in a mephitic cloud of love and romance and prudery and fastidiousness, will break through into clear sunlight as a purpose no longer to be confused with the gratification of personal fancies ...