@Zacharý It's actually quite a bit more than that and the sign usage can vary based on what level of pragmatic discourse you're talking about. Most generally, you are right; felicity is pretty much defined as 'appropriateness relative to a context'.
But at a level of abstraction/complexity/scrutiny higher, an infelicitous statement is one that violates one of the four Gricean maxims, with the assumption that what you're analysing, be it a discourse or conversation or utterance, follows Grice's Cooperative Principle. You can look these up to learn more.
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