This is the oath made in court before a witness is due to testify or give evidence, usually sworn on a bible or other holy item. The idea is that, once the witness has taken this oath, they must tell the truth or be struck down by the righteous wrath of The Lord (or at least be charged with perjury).
But why is the phrase so long? Surely by saying "the truth", the phrases "the whole truth" and "nothing but the truth" are made redundant? After doing a little research, I've come up with nothing but a couple of theories (ergo, this isn't fact, but speculation):
- Repetition for Emphasis -- Presumably, the more times "the truth" is mentioned, the more likely that the importance of the matter has been conveyed. Rather than say "I swear to tell the truth", the witness has to say "the truth, the whole truth, and nothing but the truth"; the repetition gives the oath more weight in the eyes of those testifying and makes it sound a lot more important.
- Different Types of Truth -- This is the idea that "the truth", "the whole truth" and "nothing but the truth" refer to different aspects of the truth, which means that they are not redundant and not just for show.
Straightforwardly, "the truth" is simply what the definition says -- the correct information as far as you are aware. "The whole truth" is different to this, because although this phrase still refers to correct information, the word "whole" means that nothing must be omitted.
Finally, "nothing but the truth" refers to opinion based on truth (which may not be well-informed enough) and assumption based on truth (which may not be correct). E.g., if a man saw a red car pulling up to his house and later heard a car pulling away, he may not say that he heard the same red car leaving, as he did not see it. Regardless of whether the assumption is accurate or not, it is not fact.