Masaka (まさか) means “no way… it can't be!”: what you say when something you thought impossible is happening right in front of you.
The masked rival takes off the mask. The hero stares. まさか…！ It is one of anime's great dramatic words — and an ordinary word in daily life too.
What it actually means
まさか is about expectation. You had ruled something out completely, and now it seems to be true. On its own it is a gasp; at the start of a sentence it means “surely not…”.
まさか or うそ?
Both react to surprising news. うそ！ is quick and light — “no way!”, and often happy. まさか has more weight: “I never imagined this.” It suits shocks, twists, and things you were sure would not happen.
ケーキが ない…。 まさか、ふじさん？
The cake is gone… Fuji, it can't be you?
…すみません。
...Sorry.
まさかの…
まさかの + a noun means “the … nobody saw coming”. You will see it all over Japanese headlines and social media.
Can you use it?
Yes. With friends, まさか！ on its own is fine. With a teacher or a boss, say まさか！ ほんとうですか, or just ほんとうですか. まさか is about your own surprise, not about the other person's honesty, so it is gentler than うそ.