How come it is not 약한다 but 약하다는 뜻
It is from this sentence. “약”자는 몸이 약하다는 뜻을 나타내요.

I'm guessing the grammar is from this -(ㄴ/는)다는 뜻이에요.. so how come it is not 약한다?

Question

How come it is not 약한다 but 약하다는 뜻
It is from this sentence. “약”자는 몸이 약하다는 뜻을 나타내요.

I'm guessing the grammar is from this  -(ㄴ/는)다는 뜻이에요.. so how come it is not 약한다?

Answer

"약(弱)"자는 몸이 약하다는 뜻을 나타내요
약(弱) is an adjective.

-ㄴ/는다는 뜻이에요 is used for a verb.
-다는 뜻이에요 is used for an adjective.
-(이)라는 뜻이에요 is used for a noun.

For example, 
학(學)은 배운다는 뜻이에요.(verb)
홍(紅)은 붉다는 뜻이에요.(adjective)
천 (天)은 하늘이라는 뜻이에요.(noun)

Answer

Oh. I didn't know that. Thanks.

Answer

btw "약한다" is a slang meaning "doing drugs"

Answer

약하다 is one predicate which means to be weak.
So 약하다 + 는 뜻이에요.

In addition I'm running Korean teaching youtube channel.
Maybe it will be helpful for you.
copy and paste
https://www.youtube.com/channel/UCErPtvmn08v29X0A9uojavA

Evden çıkmadan dil öğrenme fırsatını kaçırmayın. Deneyimli dil eğitmenlerimizden oluşan seçkimize göz atın ve ilk dersinize şimdi kaydolun!