particle は - What does the "〜やしない" conjugation mean?

In episode 76 of Fairy Tail, Gildarts said this to Natsu:

本気でそう思ったら、止めやしないよ。 (honki de sou omottara, tomeyashinai yo)

Which was translated in the subtitle as:

If that's what you honestly believe, I won't stop you.

I thought "I won't stop you" would simply be "tomenai yo", so I'm kind of lost about what nuance the extra "+yashinai" conjugation carries. What kind of negative conjugation is it?

Answer

As others have said, this is probably really ～やしない, which is transmutation of ～はしない. What this suffix does is usually one of two things:

1. 
   

   It makes the verb a topic (with は) and then negate it. This is used to bring up the event described by the verb and then saying it won't happen (or isn't happening, have never happened - you get the point). From the context, this seems to be what's mainly happening here, Gildarts says to Natsu: "Well as for stopping you (as you may expect or fear I would try to) - no, I won't do that".

   
   


2. 
   
   

   It's also often used when there seems to be little intention to topicalize the verb. In such cases, I think it's mostly used as a stronger and more passionate negative than the simple ～ない.