Undoubtedly, soon We shall be casting on you a weighty word.
We will give you a heavy message.
We are about to cast upon you a weighty Word
Surely We shall charge thee with a weighty word
That We, We will throw (a) heavy saying/word/declaration on you
Pretty soon you will be given the rest of Qur’an: a mighty revelation [for mankind]
Indeed We shall soon ordain a heavy responsibility upon you
Verily, WE are about to charge thee with a weighty Word
Soon We shall send down to you a burdensome command
Verily, We are soon going to charge you with the enormous and important responsibility of (conveying to mankind) the Message (of your Lord)
Verily, We shall send down to you a weighty Word (i.e. obligations, legal laws, etc.)