The meaning which I know is so changeable, I assume. The basic meaning is this(In my opinion)

1. As an adverb: Do or say something roughly and carelessly.

• 이거 너무 막 한거 아니야? (Didn't you do(or make) this too roughly?)

• 너무 막 뭐라고 하지마. 그에게도 사정이 있을거야.( don't say badly(or roughly) too much. I think he has his own situation)

2. Depending on situations, "막” can be an adjective.

• 막말 하지마. 없애버릴꺼야.(Watch your language.(or don't say heedlessly) I'll eliminate you.

3. "막” can be do, or say about something a lot simultaneously or without a calm

-After writing the answer, I became uncertain about it. The form of "막” could be something else. I know neither Korean grammar nor English grmm. I answered on your question though. Regard mine as an opinion from an ordinary Korean.

*Just saying personal experiences, people usually did "막” when they were in a hurry or tired or got tiresome so that they wouldn't like to dedicate on something. Really, "막” is being used a lot of situations. I don't think I can answer all the variations of the usage too. And indeed, seems that none of books has this information.