"懵b" is pronounced "měng bī", meaning "confused", "at a loss". The slang term originates from "懵懂" (měng dǒng), which means "muddleheaded", "ignorant".
“懵”念 meng 第三声，表示“一时的心乱迷糊”。其实这个字我们经常使用在一个词里——“懵懂”。
Actually, there are lots of confusing scenes in our life. There's no toilet paper after you take a dump; you're suddenly asked to answer a question when you fall asleep in the class; when Lee Se-dol found he's defeated by AlphaGo.
其实咱们生活中有好多“懵 b”的场景，比如，上完厕所发现纸盒里没纸了。课上睡觉，突然被老师点名起来回答问题。围棋大师李世石被 Google 的“阿尔法狗”战胜后的一脸懵 b。
Here are seveal ways to explain "懵b" in English.
1. Confused 迷惑的
I was completely confused after hearing his explanation.
2. Overwhelmed 不知所措的
Do you ever feel overwhelmed by happiness that came in a sudden?
3. Like a deer caught in the headlights 像一只被汽车大灯照到的鹿
The speaker on the stage suddenly found his pants were unzipped, which made him like a deer caught in the headlights.
台上的演讲者突然发现自己的裤拉链没拉，这让他整个人都懵 b 了。
4. Blankly 茫然地；毫无表情地；茫然若失
Facing such a question, most people will shake their heads blankly.
5. At a loss 无所适从，不知所措
He suddenly bacame at a loss, when he realized that his hidden files on computer had been discovered by his parents.
6. Mengbility 懵逼
Like zhuangbility, niubility and geilivable, menbility is also a coined word created by imaginative Chinese net users.