That's a spelling mistake.
Forth is an actual word though, and it was used improperly.
But, based on the context, it is clear to see that it is a spelling mistake, not a grammar mistake.
A true Jhin master would never take a penta
Team: Why the $@$#@%@ didn't you ult to kill the last one?!
Because...
http://img02.deviantart.net/2f4e/i/2016/038/f/7/jhin_meme_by_mikouchan-d9qyb6a.png
1-2-3- . . . Fffffffffffffffffffffff-
And then you have to reload and lose your penta. 
That's the plan, Jhin doesn't want pentas.
It is a definition of art my feeble support-like mind cannot understand.
All we know if, "Who's a good ADC? Who is? That's right, you are! You're a good ADC! Does my ADC want a kill? Oh, do they? Oh, I think they do! I think they do! Good ADC, you eat that Varus who thinks he is good outside of ARAM!"