Almost always written purely in hiragana,
まさか is a combination of まさ, a 名詞（めいし） (noun) meaning ‘precise’, or ‘exact’, and the 副助詞（ふくじょし） (adverbial particle) か. In modern Japanese,
まさか is thought of as a single 副詞（ふくし） (adverb) meaning ‘it can’t be true’, ‘no way’, or ‘don’t tell me’.
As it is treated primarily as an adverb,
まさか will most often appear at the beginning of a sentence, before some statement about what the speaker is shocked/surprised about.
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まさかタナカ君（くん）もこのジムに通（かよ）っているとは思（おも）っていなかったよ。
I never would have believed that you also came to this gym.
まさか彼（かれ）がキャプテンなんて。
Don’t tell me that he is the captain!
まさ comes from 正（まさ）, meaning ‘precise’ or ‘actual’. However, this noun is very rarely used in modern Japanese. まさに is an adverb used to mean ‘precisely’, while
まさか is another adverb used to mean ‘no way!’. However, the literal meaning of
まさか is closer to ‘is it true?’, or ‘is it actually?’, due to か’s role in creating questions.
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全然（ぜんぜん）待（ま）っていないですよ。僕（ぼく）も今（いま）まさに着（つ）いたところです。
I didn't wait at all. I actually just arrived now.
まさかリアがフランスに帰（かえ）るって本当（ほんとう）なの？
Is it actually true that Lia is going back to France?