it's funny because "18번" when you talk about 노래방 means your favorite song to karaoke ^_^ i never knew it is shorthand for the cuss word :D until now haha

시발(십할,시팔,씨팔=f*ck,damn) is a commonly used swear word. people says that to themselves when upset. people call other people 시발놈 to insult. Some indecent koreans (specially young people these days) speak this word like a habit.

as I have accidently seen on the website abt 18, however, i dont understand at all. 18 – the F word (from [bad word] that sounds like 십팔)

bad word (F) = 시발 but some of korean write 시팔, 씨발, 씨팔, etc. and 십팔 =18 its different. but maybe because the pronouncation is a bit similiar, they write 십팔 maybe? kkkkk

Your question is very unclear, can you ask it a different way? I don't see how 18 is related to anything in the body of the question you posted