Is there a problem with saying I promise to add emphasis to your words? For example can I say “you’re going to love this, I promise!” Or “I promise, I’ve never had better food than this!”
I promise is not like swearing, however you still have to be careful not to lie. Therefore, if it is evident from what you are saying that you are not really serious about what you are saying and it is only for emphasis then it is not sheker. As a side point, it is a good idea to remove the term “ I promise” from your terms of speech, because there are times that it can get a person into trouble, such when saying “I promise I will do this”.