Many sources will tell you that piano intervals are deliberately tuned "slightly out of tune". This is explained on the basis that tuning musical scales requires something called temperament. This is what Bach was referring to, in the title of the Forty Eight, although it should not be assumed that Bach was tuning his clavier in the same way we tune the modern piano.

Now take a breath.

If musical intervals have to be slightly out of tune in order to be well tuned, then logically they must be poorly tuned when they are in tune. 

Normally, saying an interval is "in tune" implies that the interval is well tuned. So it follows from the preceding paragraph that normally, an interval is out of tune if it is in tune, and in tune if it is out of tune. Logically, this means that being in tune and being out of tune are one and the same.

So why bother having your piano tuned when it is out of tune, since it is already in tune?

This reality is that "in tune" or "out of tune" are just terms that mean whatever you want them to mean. 

Most people, when they say an interval is "out of tune", mean that it does not sound right. This could be because the note pitches do not sound right, or because the tone of the interval does not sound right.

"In tune" and "out of tune" are fuzzy "made up" terms. They do have a place, namely, in describing something you do not have to define, but neverthelesss recognise when you hear it. 

They have no place in defining how a piano is tuned.