CoreanBigSis explained it well. I'd just like to inform you can use it to make what you've just said less serious. Quite a teen's language.

막 이래 means '막 이러더라(somebody says or act like this), so it is after some saying or behaviour first than we say that. how does it mean like that? Let's look into it. 막(short form for 마구): 1. strongly, aggressively 2. roughly, randomly 이러다(short form for 이리하다): 1. do like this 2. talk like this so 막 이래 means 'someone does do(talk) like this strongly(or roughly, it depends on situation, can be either way) It becames popular lingo among young girls these days actually. Let's see examples. 1. 엄마: 왜 방이 이렇게 지저분하니? 소꿉놀이하고 엄마가 치우라고 했잖니?(Mum: Why is your room is so messy? I told you to clean up after you play house. 딸: 엄마, 내가 소꿉놀이하자고 조르니까 오빠가 귀찮다고 저리가라고 소리치더니, 막 이래놨어. Daughter: Mum, I asked to bro to play house together and then he shouted at me not to bother and go away. and he did like this to me(he made this mess in my room). this is the traditional way to say 막 이러다. 2. 미진: 잊어. 니가 더 아까워. 걘 생긴 것도 별로고 성격 파탄이잖아. 넌 예쁜 데다가 착하고..Mijin: Forget about that bastard. You could do better than him(you deserve better than him). He's not good looking, and also has personality disorder. But YOU are so pretty and good hearted. 지희: 맞아. 내가 좀 아깝다. 얼굴도 소녀시대 뺨치고, 성격은 천사잖아. 막 이래(멋쩍게 웃는다). Jihee: Yes, you are so right!! I think I deserve better than him. I look great like the girl in 'girl's generation and I am sweet like an angel. I talk like this for myself(and she smiles bashfully and a bit awkward) yeah..such as that. girls use this a lot. sometimes without special meaning they put that sentence after whatever they say. but dont use it too often, cuz it can make you seem so shallow. :) have a great weekend. hope it could be helpful to you.

막 ==just 이래==since just since 막 이래 i think i'm not sure