Hi. I'm a native Korean, but not good at English.
The "마냥" has various means in use.
First, native Korean use that as "such as", "like"
Example 1 ) 너는 마치 원숭이인것 마냥 행동하는구나. - You behave like a money.
But Korean linguists define that as "forever". So, if you search the "마냥" in dictionary , you can see "forever".
Example 2) 나는 언제까지고 그것을 피할 수 없다. - I can't avoid it forever
In my opinion, second example is not often to use in formal language.
Because, in my Korean dictionary, "마냥" is not right expression in current Korean specking.
Moreover, it is slightly hard to use. (I think I use "마냥" in one time per day)