supposedly, supposably

The adverbs supposedly and supposably are both genuine words, but they have very different meanings and uses. In virtually all cases, it is the adverb supposedly that is needed.

Supposedly

Choose supposedly if you are talking about something believed, rumoured or reported:

• Five centuries ago, the world was supposedly flat.
  [It was believed the world was flat.]
• He supposedly stayed after school for tutoring.
  [It is rumoured that he stayed.]
• A sea serpent supposedly lives in Lake Memphremagog.
  [It is reported that a sea serpent lives there.]

Supposably

Supposably is almost always used mistakenly (in place of supposedly) and should therefore be used with caution, if at all.

In some dictionaries, supposably is listed as the adverb form of the adjective supposable; it therefore means “conceivably.”

• What could supposably go wrong?

However, because supposably is rarely used correctly, it is best to avoid it. You can replace it with conceivably or possibly:

• What could conceivably (or possibly) go wrong?