In 11:25 of episode 6, there is a bleep sound, but I don't know why is it being censored. Jibril and Steph are surprised by it, so would it be a dirty joke or something else?
There is a reason why Jibril and Steph are surprised in this situation. As you should know Steph and Jibril are girls, so the question would be "what is a girl's nether region?". It is obvious that a girl's nether region would be their:
From otaku - Censorship:
As this is considered as a dirty joke, it is censored because
ま☆こ in direct translation does mean "Nether Regions". If there is no censorship in this scene, it would violate "Japanese Censorship Law". When it is uncensored the word would be
まんこ (romanized as
"manko") which directly means vagina.
Hence, the censorship in episode 6 of No Game No Life.
The Japanese text there is ま★こ ma*ko, which (to someone who knows Japanese) is obviously a censored form of まんこ manko, which does refer to the female genitalia, as Sukeibe points out.
The key insight here is that manko is in fact vulgar slang for the female genitalia, which is why you will see it censored. The translation "nether regions" fails to capture this vulgarity, I suppose because it's constrained by the rules of word chain (can you think of a vulgar term for the female genitalia that begins with "n" and ends with "s"? I can't...). If a technical/medical term for the female genitalia were used instead (e.g. 女性器 joseiki lit. "female sex organ"), it would probably not have been censored.