Is an unhappy smile the same as a frown? I tried to google images of an unhappy smile and that's mostly what I found. I don't think an unhappy smile is the same thing as a frown, but I could be wrong. What is an unhappy smile? A description or picture would be helpful.
An unhappy smile is still a smile, insofar as the corners of the mouth are drawn back or upturned. However, the eyebrows may be furrowed, and the eyes may be squinched or teary. Also, the lips tend to be pressed together, rather than relaxed as in a happy smile. Here is an example:
The phrase can also indicate that the subject is forcing a smile or putting on a happy face, as demonstrated in this passage from Wolfsden, published in 1856:
Although it can certainly also mean that someone has an unattractive smile — this is a very modern meaning, arising with the increased popularity of cosmetic dentistry.
An unhappy smile is certainly not a frown.
An unhappy smile is when you try to smile even when you are not happy. From the look of your face, it might seem that you are smiling, but you are really not happy.
You can say that an unhappy smile is a smile that is on your lips, but not in your eyes.
Also see this question.