Fetching details from the dictionary...

by no means; never!; well, I never!; you don't say!; no way!
certainly (not); surely (not); hardly (expected)
something unexpected; emergency
indeed; really; truly; unexpectedly
まさか君がここにいるとは思わなかった。
I never expected you to be here.
まさか彼が遅刻するとは信じられない。
I can't believe he would be late.
まさかの結果に言葉が出ない。
I am speechless at this unexpected result.