An old question, for which I could find absolutely NO reference on the internet for many years.. perhaps I wasn't looking in the right places, but I have no clue about its usage either.
Then I stumbled upon this article just now that actually happened to address the same premise. I tried to figure out myself as a function of context but my better judgement was to seek expert advice. So here goes..

In the following excerpt I found in an article,
(http://www.urbandictionary.com/define.php?term=valium&defid=1845831)

Ricky: Yo player let me get some of that valium! 
Albert: **** off these are for my back spasms. 
Ricky: Okay fine then I will tell your girlfriend about last weekend. 
Albert: Fine, here you go a**hole. 
Ricky: Word.

What exactly does 'word' mean at the end? What are its connotations? Thanks :)