WC: Water Closet n. 盥洗室，廁所

廁所
Some of the ways we refer to the place where you relieve yourself in English include:
Toilet - Any place that has an actual toilet. 馬桶
Restroom - Often in a restaurant or public place. 公共場所的廁所
Bathroom - Inside someone's house. 家里的廁所
Men's room or ladies room 男士，女士衛生間

大小便
Some of the idioms we use in English to express that we need to relieve ourselves are:
Hit the head
Hit / Use the John
Porcelain throne
小號 drain the main vein 僅限男生用，略微粗魯
大號 Dropping a deuce
大號 Take a dump

放屁
There is also several ways of saying fart in English, some are more acceptable than others.
相對較禮貌
To pass gas
Toot
Breakwind
較粗魯
Rip one
Cut the cheese
粗魯
Shart -- When you think you have to fart but you actually have a little poop come out.

問廁所
口語化，適用任何場景，不會太正式也不會太粗魯
D- "Excuse me miss where can I find the restroom?"
C- "It's the third door on your left sir"
比較隨意，和親近的朋友使用
D- "Hey John, where is your toilet?"
C- "Just down the hall."
非常正式
D- "Is there a place where I can relieve myself"
C- "Yes, the second floor has a lavatory"