1.
surely not, no way, by no means
Expresses disbelief or denial that something could be true or happen.
2.
just in case, in the unlikely event
Used as まさかの before a noun for an unlikely emergency (まさかの時, 'a rainy day'). In current use まさかの also means 'unexpected, shocking' (まさかの逆転負け, 'a shock come-from-behind defeat').
Common patterns
- まさか〜ないだろう (surely...won't)
- まさか〜とは思わなかった (never thought that...)
- まさか！ (No way! - as an exclamation)
- まさかの時 (in case of emergency)
- まさかの事態 (unexpected situation)
Usage
まさか conveys strong doubt or disbelief. When something actually happens that you thought was impossible, you might say まさか本当だとは... (I never thought it was actually true...).