Question: I recently have had a discussion about my friend's tone in his email.

His supervisor asked him whether he wants to attend a summer school in May. He wanted to thank him for his invitation and also wanted to keep the mail short. As a result, that is what he came up with:

> 
> Hello Professor,
> 
> 
> I have plans during that period. Thanks, but no thanks.
> 
> 
> Regards,  
> 
>  John 
> 
> 
> 

He is getting along well with his supervisor, but I believe that the phrasing is a bit disrespectful nevertheless. It might be the case that I am not a native speaker. 

Am I exaggerating the situation and this is something can be overlooked?

Answer: 
Given that *"thanks, but no thanks"* is often used as a mocking euphemism for *"gee, that's a stupid idea, I will of course not do that"* I would suggest not using it on your advisor.

What's wrong with just writing "thank you, but unfortunately I won't be able to make it for reasons A and B"?
If I was invited to attend something that I couldn't I would simply say, thanks for inviting me, but I'm sorry I'll have to decline, as I'm busy that day.