I saw this sentence in the subtitle. However, I am unsure if it is correct grammatically.
I dreamt I was snorting blow off of Tom Jones' ass.
This sentence is grammatically correct.
As for the meaning, to snort blow means to inhale cocaine through ones nose. This is usually done by putting the cocaine in a line on a surface, before inhaling it through a tube-like object, like a rolled-up bill.
In this case, the character dreamt that she was using Tom Jones' ass as surface to snort the cocaine off.
It's correct, although I can't help but wonder how accurate the Chinese translation is.
Let's break it into parts. The outer part is "I dreamt X".
I dreamt [I was a butterfly].
The inner part is "I was doing X some place Y":
I was [eating dinner] at [my friend's house]
I dreamt I was eating dinner at my friend's house.
Substitute "snorting blow" for "eating dinner", and "off Tom Jones' ass" for "at my friend's house" and you have the sentence from the TV program.