[orig. from SF's fanzine community, popularized by Jerry Pournelle's column in BYTE]
1. Supposed to be available (or fixed, or cheap, or whatever) real soon now according to somebody, but the speaker is quite skeptical.
2. When one's gods, fates, or other time commitments permit one to get to it (in other words, don't hold your breath). Often abbreviated RSN. Compare copious free time.