The world 'Narsaye' was originated from an English rap group called D-Block Europe that promoted the word in there songs and active slang between one another causing the word to explode and become a very popular slang.
The word defines as 'You know what I'm saying'. The term is used to agree or say you get me but is more of a joke rather than being said seriously in context.
LB : 'Thats some good loud'
YOUNG ADZ : 'Narsayeeeeeeee'
