"There's nothing to tell" is a response to someone's asking for details about an event or a story, where the responder implies that the information the asker wants to know doesn't actually apply to the event or story.
For example:
Q: "What happened at the party last night?"
R: "There's nothing to tell."
(Nothing happened or things that you don't really care about happened, but that's it - i.e. I hung out for five minutes and went home.)
The responder could also say, "There's nothing to tell," in order to downplay what actually happened.
For example:
Q: "It must've been really hard to
become such a good swimmer. How did
you do it?"
R: "There's nothing to tell."
(The answer is too long and/or boring, so I'll spare you the details.)
Another instance is your own suggestion of using "There's nothing to tell" to mean, "I don't want to tell you what happened."
"There's nothing to say" is very similar to "There's nothing to tell" in that the responder thinks any response the asker might expect either doesn't apply or doesn't matter. It's also possible that the responder doesn't really want to answer the question. The biggest difference is that "tell" generally involves some sort of story or narrative, and "say" involves some sort of issue or non-narrative topic.
For example:
"I would love to visit Area 51.
Wouldn't you?"
"There's nothing to say."
(I don't think it exists, so my response doesn't apply.)
"Do you think signing that bill into
law was the right decision?"
"There's nothing to say."
(It doesn't matter what I think.)
I think you’re on the right track. It is evident that the words are of the same origin. However, in terms of connotation, they have gone in separate directions. Take the word hypocrite, for example. On the one hand, we would tend to collocate hypocrite with specious, false, and sophism.
Yet looked at literally, being sophisticated is synonymous with being a hypocrite. That is, if we understand both as the possession of refined education (as found among courtiers) and devoid of naïveté.
One who is naïve will hardly be considered sophisticated or a hypocrite. Whereas it is a mark of sophistication to be a hypocrite (in the literal sense, as demonstrated by courtiers). Hiding one's true motives and feelings is both a mark of hypocrisy and sophistication. Only the naïve (those lacking in sophistication / hypocrisy / sophism) think it wise (the root meaning of the term — Sophia (Σοφíα, Greek for wisdom) to show true feelings and motivations at all times.
Who among us would question the wisdom of deception, at times?
So, the bottom line is that your conclusion is correct. It’s just that in the one case, the negative is emphasized, while in the other positive aspects are given greater weight: true hypocrisy in its literal sense.
Best Answer
From the OED:
caregiver n. orig. U.S. a person, typically either a professional or close relative, who looks after a child, elderly person, invalid, etc.; a carer.
caretaker n. a. One who takes care of a thing, place, or person; one put in charge of anything.