まさか

From Old Japanese. First attested in the Man'yōshū, compiled 759 CE.[1]

There are various theories regarding the ultimate derivation:

• Being the initial ma- is from 目 (ma, me, “eye”), it was thought to be an alteration of 目前 (mesaki, “before one's eyes”), with the me- element becoming ma-, but there was no attestation of masaki.[1]

• Compound of root 正 (masa, “exactly”, found in compounds such as まさに (masa ni, “certainly, exactly, without doubt”)) +‎ か (ka, “special suffix indicating condition”).[1][2]

まさか • (masaka) 

1. unexpectedly, inconceivably, impossibly, surely

   まさか事(じ)故(こ)に遭(あ)うとは思(おも)ってもいませんでした。

   Masaka jiko ni au to wa omotte mo imasen deshita.

   Never in my wildest dreams would I have imagined that I would be in an accident.

まさか • (masaka) 

1. of course not, don't be silly

   まさか！そんなわけないでしょう？

   Masaka! Sonna wake nai deshō?

   Goodness, of course not!

2. no way, come off it

   まさか、本(ほん)当(とう)にやったのか？

   Masaka, hontō ni yatta no ka?

   No way, he/she actually did it!?

3. (The addition of quotations indicative of this usage is being sought:) no shit

まさか • (masaka) 

1. an unexpected event

   当日(とうじつ)にまさかの事件(じけん)

   tōjitsu ni masaka no jiken

   unbelievable incident on the day of

   まさかの時(とき)

   masaka no toki

   time of need